.gitignore: clean up and document

- Remove Rust-related files since we don't use Rust in this repo.
- Make `__pycache__` pattern more generic so that it covers both `src/`
  and `src/cosalib/`.
- Delete `*.pyc` and `*.pyo` patterns since they're covered by the now
  more generic `__pycache__` pattern now.
- Delete obsolete `maipo` pattern.
- Delete `.gocache` pattern which was used by `bottlecap`, which is no
  longer in this repo.
- Delete `.idea` pattern for the IntelliJ IDE. Developer-specific
  patterns should live in the developer's global `.gitignore` file or
  repo-local exclude list instead.
- Make `bin` and `.coverage` patterns anchored at repo root.
- Add comments for remaining patterns.

.gitignore

@@ -1,12 +1,11 @@
-/Cargo.lock
-/target
-**/*.rs.bk
-src/__pycache__/*
-.gocache
-maipo/
-*.pyc
-*.pyo
+# generated by Python
+__pycache__/
+
+# generated by `make shellcheck`
 *.shellchecked
-.coverage
-.idea
-bin/
+
+# generated by `pytest`
+/.coverage
+
+# generated by `make all`
+/bin/