docs: write real narrative pages, weave guides into the toctree

[cailmdaley]

What & why

The deployed developer docs read as blank because the four hand-written Sphinx pages (index, about, installation, quickstart) were still the unmodified python-package-template placeholders. The Sphinx machinery is fine — autodoc populates the full API reference, the awesome theme renders, gh-pages deploys. This PR writes real content for the front pages, wires the existing real guides into the navigation, and adds a docs-build CI check.

Docs content

The four template pages, rewritten with grounded content:

• index — a true landing page: what sp_validation is, the four tasks (shear validation → post-processing → cosmology validation → inference), the pipeline flow chart, where to go next.
• about — CosmoStat / UNIONS context, the problem the package solves, authors & contributors (from the README), contact, MIT license.
• installation — container / Apptainer first (recommended), then a uv dev checkout. Removes the false pip install sp_validation PyPI claim (the package is not on PyPI).
• quickstart — a deliberately light four-stage orientation, each stage linking to its guide. Kept brief pending a broader docs rework.

Navigation & the existing real guides:

• toc.rst regrouped into Getting Started / User Guide / API Reference / Guidelines.
• run_validation.md and post_processing.md were stray .md files in no toctree — built by Sphinx but unreachable from the sidebar. They (and the Leakage tutorial, moved out of "API Documentation") now live under User Guide.
• Added H1 titles to the two header-less guides; fixed post_processing's broken internal cross-reference and two H2→H4 heading jumps in the Leakage tutorial.

Cleanup:

• contributing.rst: fixed dead github.com/martin.kilbinger/... links → CosmoStat/sp_validation on develop.
• Removed citing.rst + its placeholder my_ref.bib — nothing substantive to cite yet.

Docs CI

Split the docs build into its own deploy-docs.yml workflow:

• Builds on every pull request (as a check) and on pushes to develop.
• Deploys to gh-pages only on develop pushes.
• Runs inside the published image (which carries the scientific stack autodoc imports) and installs the checked-out package on top via .[docs], so the rendered docs reflect the code under review.
• The now-redundant deploy-docs job is removed from deploy-image.yml.

Previously the docs only built on develop, so a broken docs build never surfaced on a branch or PR. This PR is itself the first to exercise the new check.

Verification

Built the way CI does — sphinx-apidoc + sphinx-build inside ghcr.io/cosmostat/sp_validation:develop (sphinx 8.2.3, full scientific stack). Build succeeds with zero warnings; the API pages populate with [source] links (autodoc imports all resolve), and the rendered landing page shows real content.

Docs redeploy on merge to develop; please don't merge until reviewed. A broader docs rework is the natural follow-up — this PR stands up the structure.

(Pre-existing docstring RST warnings were fixed separately, straight to develop, in 78b4d17.)

— Claude on behalf of Cail