docs: update install, contrib, and usage documentationI6f4b5f5ca73780cecf66a553cce80aa3f75df2ce

This change refactors documentation, especially //:README.md,
//:CONTRIBUTING.md, and centralized most of the rest of the contributing
and usage documentation into `//doc`.

Documentation changes focus on cleaning up erroneous language,
reformatting, and restructuring docs in order to reduce visual noise -
the goals are to enable users and contributors to find what they're looking
for much more quickly, reduce (sometimes erroneous) duplication, and
present a structured approach for where to put docs.

Additionally:

- Added CI pipeline badge for the `trunk` workflow in `//:README.md`
- Converted embedded emoji characters to GitHub's emoji syntax
- Moved most sections with image and installation instructions into
  `<details>` elements
- Replaced references to gitter with matrix
- Introduced `treefmt-nix` to `devShells.default` in the flake, and all
  non-generated documentation was formatted with `nix fmt` (running
  `mdformat` under the hood)
- Regenerated //doc/md and //doc/man and refactored inline docs from various
  pages that were iterating out command line details so that they simply
  point to //doc/md instead
- Rewrote contributing docs to focus on using `nix`, to ensure that the
  development shell is used. This enforces consistency across different
  platforms with the revision and configuration of each tool we use, and
  simplifies the onboarding story for contributors -- they only need to
  install and configure nix (and optionally direnv). A dev container
  will be provided in the future [0] as an alternative for users on
  Windows (who can use WSL) or want to avoid installing `nix`.

[0]: https://github.com/git-bug/git-bug/issues/1364

Closes: #1212 #1276 #1330 #1357
Change-Id: I6f4b5f5ca73780cecf66a553cce80aa3f75df2ce

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