Document user configurable settings (#1788)

Resolves #1573

Since winning the war on Ansible, we've dropped support for many
TinyPilot settings that were configurable via
`/home/tinypilot/settings.yml`.

Apart from this blog post
([/whats-new-in-2022-05/](https://github.com/tiny-pilot/tinypilotkvm.com/blob/ba842c50235005fb9253e316cf4f85a543548634/content/blog/whats-new-in-2022-05/index.md))
we haven't really documented (publicly) which settings are supported,
deprecated, and unsupported. This PR does just that:
* Document preview:
[docs/user-configurable-settings.md](https://github.com/tiny-pilot/tinypilot/blob/document-user-configurable-settings/docs/user-configurable-settings.md)

### Notes
1. The list of [supported
settings](https://github.com/tiny-pilot/tinypilot/blob/document-user-configurable-settings/docs/user-configurable-settings.md#supported-settings),
were compiled from
*
[app/update/settings.py#L76-L86](https://github.com/tiny-pilot/tinypilot-pro/blob/78269651ca646746b7b55d83d19b377a943ec7de/app/update/settings.py#L76-L86)
3. The list of [deprecated
settings](https://github.com/tiny-pilot/tinypilot/blob/document-user-configurable-settings/docs/user-configurable-settings.md#deprecated-settings-legacy),
were compiled from a combination of
*
[debian-pkg/opt/ustreamer-launcher/launch#L95-L101](https://github.com/tiny-pilot/tinypilot-pro/blob/78269651ca646746b7b55d83d19b377a943ec7de/debian-pkg/opt/ustreamer-launcher/launch#L95-L101)
* "Unsupported vars" from
tiny-pilot/tinypilot-pro#972 (comment)
4. The list of [unsupported
settings](https://github.com/tiny-pilot/tinypilot/blob/document-user-configurable-settings/docs/user-configurable-settings.md#unsupported-settings-non-configurable),
were compiled from a combination of
*
[2.6.0/ansible-role/defaults/main.yml](https://github.com/tiny-pilot/tinypilot-pro/blob/2.6.0/ansible-role/defaults/main.yml)
*
[2.6.0/ansible-role-ustreamer/defaults/main.yml](https://github.com/tiny-pilot/tinypilot-pro/blob/2.6.0/ansible-role-ustreamer/defaults/main.yml)
*
[blog/whats-new-in-2022-05/index.md#enabling-h264-video](https://github.com/tiny-pilot/tinypilotkvm.com/blob/ba842c50235005fb9253e316cf4f85a543548634/content/blog/whats-new-in-2022-05/index.md#enabling-h264-video)

### Resources used
* https://tinypilotkvm.com/pro/changes#261
* tiny-pilot/tinypilot-pro#972
*
[blog/whats-new-in-2022-05/](https://github.com/tiny-pilot/tinypilotkvm.com/blob/ba842c50235005fb9253e316cf4f85a543548634/content/blog/whats-new-in-2022-05/index.md)
*
[2.6.0/ansible-role/defaults/main.yml](https://github.com/tiny-pilot/tinypilot-pro/blob/2.6.0/ansible-role/defaults/main.yml)
*
[2.6.0/ansible-role-ustreamer/defaults/main.yml](https://github.com/tiny-pilot/tinypilot-pro/blob/2.6.0/ansible-role-ustreamer/defaults/main.yml)

<a data-ca-tag
href="https://codeapprove.com/pr/tiny-pilot/tinypilot/1788"><img
src="https://codeapprove.com/external/github-tag-allbg.png" alt="Review
on CodeApprove" /></a>

docs/user-configurable-settings.md

@@ -0,0 +1,66 @@
+# User Configurable Settings
+
+It is no longer possible to make changes to TinyPilot's undocumented settings through the `/home/tinypilot/settings.yml` files. This document outlines which settings are supported, deprecated, and unsupported.
+
+## Supported Settings
+
+The following settings are supported and remain configurable through `settings.yml`:
+
+- `janus_stun_port`
+- `janus_stun_server`
+- `tinypilot_external_port`
+- `tinypilot_external_tls_port` (Pro only)
+- `tinypilot_manage_tls_keys` (Pro only)
+  - Whether TinyPilot manages TLS keys. Users can override this setting if they want to [provide their own TLS keys](https://tinypilotkvm.com/faq/own-tls-key).
+- `ustreamer_desired_fps`
+  - Desired frames per second. Defaults to 30 when not set.
+- `ustreamer_edid`
+  - EDID for TC358743 chip.
+- `ustreamer_h264_bitrate`
+  - Set the bitrate in Kb/s for the H264 stream (e.g., 2000). The range of allowed values is [25, 20000]. Defaults to 5000 when not set.
+- `ustreamer_quality`
+  - Quality of the JPEG encoding from 1 to 100 (best). Defaults to 80 when not set.
+
+## Deprecated Settings (Legacy)
+
+The following settings are still configurable through `settings.yml`, but we may remove configuration support for them in the future:
+
+- `tinypilot_keyboard_interface`
+- `tinypilot_mouse_interface`
+- `ustreamer_drop_same_frames`
+  - Number of same frames to drop.
+- `ustreamer_encoder`
+  - Encoding method to use, such as `m2m-image`.
+- `ustreamer_format`
+  - Device input format, such as `uyvy`.
+- `ustreamer_resolution`
+  - Stream resolution, such as `1280x720`.
+- `ustreamer_use_dv_timings`
+  - Whether to use Digital Video (DV) timings.
+- `ustreamer_workers`
+  - Number of worker threads to use.
+
+## Unsupported Settings (Non-configurable)
+
+The following settings are unsupported and no longer configurable through `settings.yml`:
+
+- `tinypilot_debian_package_path`
+- `tinypilot_enable_debug_logging`
+- `tinypilot_install_janus`
+- `tinypilot_interface`
+- `tinypilot_port`
+- `ustreamer_brightness`
+- `ustreamer_capture_device`
+- `ustreamer_compile_janus_plugin`
+- `ustreamer_debian_package_path`
+- `ustreamer_edids_dir`
+- `ustreamer_h264_sink`
+- `ustreamer_h264_sink_mode`
+- `ustreamer_h264_sink_rm`
+- `ustreamer_interface`
+- `ustreamer_persistent`
+- `ustreamer_port`
+- `ustreamer_repo`
+- `ustreamer_repo_version`
+- `ustreamer_tcp_nodelay`
+- `ustreamer_video_path`