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branch was a manual step in the release process and often out of date.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/_templates/page.html

@@ -1,17 +1 @@
 {% extends "!page.html" %}
-{% block body %}
-{% if current_version and latest_version and current_version != latest_version %}
-<p class="scv-banner scv-sphinx_rtd_theme">
-  <strong>
-    {% if current_version.is_released %}
-    You're reading an old version of this documentation.
-    If you want up-to-date information, please have a look at <a href="{{ vpathto(latest_version.name) }}">{{latest_version.name}}</a>.
-    {% else %}
-    You're reading the documentation for a development version.
-    For the latest released version, please have a look at <a href="{{ vpathto(latest_version.name) }}">{{latest_version.name}}</a>.
-    {% endif %}
-  </strong>
-</p>
-{% endif %}
-{{ super() }}
-{% endblock %}%