docs: simplify build + zrepl.github.io publishing (#913)

This PR simplifies how we build and publish docs:

- **Publish from `master` branch, retire `stable` branch.** The `stable`
branch was a manual step in the release process and often out of date.
Docs are now built and published directly from `master`.
Release-specific docs are available in the `zrepl-noarch.tar` asset on
each GitHub release.

- **build dependencies**: use `uv` for dependency management

- **zrepl.github.io: retire multi-version docs**: before this PR we used
`sphinx-multiversion` to publish multiple docs versions to
`zrepl.github.io`. This was never worth the pain, so, this PR removes it
in order to simplify stuff. Old docs are available in the GitHub
releases, and the docs now have a version dropdown that links there for
a hand-curated set of versions.

- **GitHub pages repo checkout**: use HTTPS because that's what I use
these days for all things GitHub. Switch CircleCI to a fine-grained PAT.

Refs
- docs bug https://github.com/zrepl/zrepl/issues/895
  - links to config examples should work again after this PR

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/changelog.rst

@@ -48,6 +48,7 @@ INCOMPLETE LIST OF CHANGES
 * |maint| Update to Go 1.25 toolchain with Go 1.24 language level.
 * |maint| Fix deprecations exposed by the toolchain update.
 * |maint| Long-overdue update of all our dependencies & address deprecations.
+* |maint| Publish docs from ``master`` branch, retire ``stable`` branch.
 * |maint| The `make release-docker` in CircleCI produces executables that are bit-identical to my personal machine.
 
 0.6.1