docs: modernise the installation guide

[HarryCampion]

The installation page still recommended Anaconda as the way to install Loman, and told
readers to run python install graphviz — not a command — to get a Python library Loman
does not use. It made no mention of uv, of the Python versions Loman actually
supports, or of how to set the project up to work on.

What it says now

Requirements — Python 3.11+, tested on 3.11 through 3.14. The page never said.

Installing Loman — uv first (uv add, uv pip install, and uv run --with loman for
a throwaway environment), pip alongside it, and conda covered honestly: Loman is published
on PyPI and not on conda-forge, so inside a conda environment the answer is still pip.
Graphviz is on conda-forge, which is why conda still earns a mention.

Optional extras — kept from #411, with the install line shown in both uv and pip form.

Graphviz — Loman shells out to dot; the Python glue is pydotplus, and it arrives
with Loman automatically. The old text named the graphviz PyPI package, which is not a
dependency at all. Replaced with a per-platform table, a dot -V check, and Windows
PATH guidance pointing at where current installers actually put the binary.

Installing for development — a new section. Clone, make install, done. It explains
that the environment comes from Rhiza: the
Makefile is a thin shim forwarding to the rhiza-task CLI fetched through uvx, and
make install also runs local-setup.sh, which installs Graphviz so the visualization
tests can run. Then make test / fmt / all / help.

Credit

The Graphviz sections are @nikjascrazzy's work from #378, resolved against the
## Optional extras section that landed in #411 after that PR was opened (which is why
#378 now shows a conflict), with a conda-forge row added. They are credited as
co-author on the commit. #378 is superseded by this rather than rejected — happy to
reverse that if they would rather land theirs first.

Verified rather than assumed

• tests/test_docs.py — 7 passed.
• The built page contains each command as rendered text — uv add loman,
  uv pip install loman, uv run --with loman, pip install loman,
  conda install -c conda-forge graphviz, make install, make help.
• Both relative links resolve (interactive_widget, development/rhiza).
• loman is absent from conda-forge (HTTP 404) and present on PyPI (200); graphviz is
  on conda-forge (200) — so the conda advice is checked, not guessed.
• rhiza-task's install really does depend on setup, which is what makes the
  Graphviz claim in that section true.
• The build's single remaining warning is the pre-existing one in release.md
  (../../CHANGELOG.md, outside docs_dir), untouched here.

Related

The README's Development section has its own inaccuracies — it advertises make check,
which is not a target, and calls the task runner "Taskfile". Out of scope here; worth a
follow-up.

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