Site updates for v20.2.0-alpha.1

Fixes cockroachdb#7427.

Summary of changes:

- Create a new v20.2 folder, and copy the 20.1 content to it; delete the
  various "new or changed in v20.1" callouts from the copied files

- Copy includes to a new '_includes/v20.2' folder.

- Copy images to a new 'images/v20.2' folder.

- Edit image link text in the referencing documents as needed to point
  to the right places.

- Update the various YAML config files with the new version and release
  info.

- Create a new sidebar JSON file for v20.2.

- Do *not* copy over anything related to CockroachCloud for v20.2.  This
  includes editing page frontmatter to remove build directives that pull
  in certain pages for CockroachCloud.

- Update the CockroachDB local YAML config to exclude 19.2 from the
  build process (this file is used to speed up local builds).

- Various edits to the 'Upgrade to CockroachDB v20.2' docs, changing
  version references as appropriate, and removing some version-specific
  limitations.

- Update the base YAML config to use more specific version information
  as required by templates in the `install-cockroachdb-linux` etc. pages

- Miscellaneous edits to CLI output as appropriate.

- Backports: Add Eric's 'scalar expressions' update from cockroachdb#7298; port
  forward Amruta's changes from cockroachdb#7479.

- Finally: Add stub release notes page for v20.2.0-alpha.1, which will
  be filled in by cockroachdb#7426.

_config_base.yml

@@ -49,12 +49,12 @@ release_info:
     docker_image: cockroachdb/cockroach
     build_time: 2020/05/26 11:00:26 (go1.13.4)
     start_time: 2020-05-26 11:01:26.34274101 +0000 UTC
-  # v20.2:
-  #   name: v20.2.0
-  #   version: v20.2.0
-  #   docker_image: cockroachdb/cockroach-unstable
-  #   build_time: 2020/05/12 11:00:26 (go1.13.4)
-  #   start_time: 2020-05-12 11:01:26.34274101 +0000 UTC
+  v20.2:
+    name: v20.2.0
+    version: v20.2.0-alpha.1
+    docker_image: cockroachdb/cockroach-unstable
+    build_time: 2020/05/12 11:00:26 (go1.13.4)
+    start_time: 2020-05-12 11:01:26.34274101 +0000 UTC
 
 
 include: ["_redirects"]

_config_cockroachdb.yml

@@ -1,8 +1,6 @@
-homepage_title: CockroachDB Docs
-
-baseurl: "/docs"
+baseurl: /docs
 destination: _site/docs
-
+homepage_title: CockroachDB Docs
 versions:
+  dev: v20.2
   stable: v20.1
-  dev: v20.1

_config_cockroachdb_local.yml

@@ -4,6 +4,7 @@ exclude:
 - "v2.0"
 - "v2.1"
 - "v19.1"
+- "v19.2"
 - "ci"
 - "scripts"
 - "vendor"

_data/releases.yml

@@ -167,6 +167,8 @@
       no_windows: true
 - title: Testing releases
   releases:
+    - date: June 2020
+      version: v20.2.0-alpha.1
     - date: Apr 21, 2020
       version: v20.1.0-rc.2
     - date: Apr 14, 2020