docs: MkDocs site + ICA/AMICA concepts, figures, community files - #97

Closes #96.

Documentation for the JOSS-readiness track (Phase R2 in .context/plan.md): the MkDocs Material site, an intro theory section with figures, and the community-health files.

What changed

• Site infra: mkdocs.yml (Material theme, mkdocstrings with docstring_style: numpy, git-revision-date), docs extra in pyproject.toml, .github/workflows/docs.yml (build + deploy to GitHub Pages), .gitignore for site/.
• Core pages: Home, Getting Started (uv install + quickstart), User Guide (Backends & Devices, Validation & Parity), API Reference (AMICA / PyTorch / NumPy via mkdocstrings), Development, Changelog.
• Background section (new): "What is ICA?", "What is AMICA?", "How AMICA works" for readers new to ICA, with real LaTeX math (MathJax via arithmatex) and 6 figures.
• Figures (in docs/assets/figures/): ICA source->mix->unmix schematic, mixing-geometry scatter, generalized-Gaussian density family, mixture source density, AMICA algorithm-flow diagram, and a log-likelihood convergence curve from a real AMICA fit on the bundled sample EEG (final LL = -3.404, matching the Fortran-parity target).
• Community health: CONTRIBUTING.md, CODE_OF_CONDUCT.md (Contributor Covenant 2.1), CITATION.cff (with the Palmer/Kreutz-Delgado/Makeig 2012 AMICA reference).

Verification

• uv run mkdocs build --strict is green.
• All figures visually checked; the two SVG schematics and four matplotlib plots pass the figures-plugin QA.

Hosting

Once the repo is at github.com/sccn/pyAMICA, its project Pages serve at the sccn org Pages custom domain subpath -> https://eeglab.org/pyAMICA/. Links are relative; stages at neuromechanist.github.io/pyAMICA/ pre-transfer.

Notes / follow-ups

• Enable GitHub Pages (Settings -> Pages -> Source: GitHub Actions) for the deploy job.
• Content depending on Phase R1 is stubbed with notes: backend performance numbers, the k-factor figure (Data-size (k-factor) sweep: cross-backend IC equivalence vs frames at 70ch #90), and the EEGLAB drop-in guide (Drop-in EEGLAB AMICA replacement: equivalent output, no extra interpretation #92) get filled once R1 lands.
• The MIT->BSD license metadata fix is in a separate PR (Fix license metadata to BSD-3-Clause #98).

Tested

uv run mkdocs build --strict (green). Docs/meta only; no runtime code touched.