pin build deps in requirements.txt for native host build

The handbook is served natively (no Docker) on the Gitea box: a python
venv runs mkdocs build, and the reverse proxy serves the static site/ at
docs.rethinkstudios.io. Pin the build deps so the host venv reproduces the
build exactly:

  pip install -r requirements.txt && mkdocs build

Verified the host rebuild flow from scratch: fresh venv + clean
pip install -r requirements.txt + mkdocs build --strict builds all 6 pages
with no errors. Static output uses relative URLs and site_url is the docs
subdomain, so it serves correctly behind the proxy at the subdomain root.
site/ and .venv/ stay gitignored.

Signed-off-by: disqualifier <dev@disqualifier.me>

requirements.txt

@@ -0,0 +1,10 @@
+# Build dependencies for the handbook docs site.
+# Host build: python -m venv .venv && . .venv/bin/activate
+#             pip install -r requirements.txt && mkdocs build
+# Output: static site/ served by the reverse proxy at docs.rethinkstudios.io.
+
+mkdocs-material==9.7.6            # theme — pulls mkdocs, pymdown-extensions, pygments
+mkdocs==1.6.1                     # pinned explicitly for a reproducible build
+mkdocs-material-extensions==1.3.1
+pymdown-extensions==11.0          # admonitions, tabbed, superfences, annotations
+Pygments==2.20.0                  # syntax highlighting in code blocks