**What type of PR is this?**

> Documentation

**What does this PR do? Why is it needed?**

- Added examples/hello and examples/basic_gazelle. The first is a
minimal example, the second uses Gazelle. Both use modules.
- Removed examples/basic-gazelle. It served as an example for a tutorial
that's been replaced upstream. It used WORKSPACE, so it needed an
update. I replaced it with a smaller example, since it pulled in a lot
of external dependencies.
- Updated .bazelci/presubmit.yml to build examples on all platforms. I
wonder if we want to use bazel_integration_test or go_bazel_test for
this though.
- Several updates to README.rst:
- Removed Announcements section since it's a bit cluttered and doesn't
mention recent releases.
  - Fixed formatting on gomock link.
- Removed "The rules are in the beta stage of development" since they're
actually pretty mature.
- Added arm64 as a supported platform. It's being tested on macOS at
least.
- Rewrote the Setup section to link to the new user tutorial, the two
examples, and the bzlmod/WORKSPACE docs.

**Which issues(s) does this PR fix?**

Fixes #3717
Fixes #3183
Fixes #1912
Fixes #1154
Updates #1643

**Other notes for review**

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Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>