feat: add sphinx autobuild

[SamJUK]

This PR proposes to install the Sphinx Autobuild package to simplify local development of the docs. Some key features it provides are:

• Built-in Web Server
• Hot Reloading on source file change

With this, the proposed local build steps would be

git clone https://github.com/wardenenv/docs.git warden-docs && cd warden-docs
pip install -r requirements.txt
make watch

[SamJUK]

Re the failing build. Upstream image (ammaraskar/sphinx-action@0.4) uses python:3-slim which received an update to 3.13 recently which is not compatible with current Sphinx version. Options I see are:

1. Replace the GH action with a python container pinned to 3.12, and run the build steps manually.
2. Update Sphinx & the GH Action to their latest builds.

More than happy to submit another Pr if your strapped for time.