Add llms.txt and llms-full.txt to documentation site

[Copilot]

Adds llms.txt support so LLMs can discover and ingest the docs site content at mono.github.io/SkiaSharp.Extended/.

Generation

Both files are generated at build time by docs/generate-llms-full.sh:

• llms.txt — Curated index with links to all doc pages and API reference, copied from docs/llms.md source template
• llms-full.txt — Concatenation of docs/llms-full-header.md + index.md + docs/*.md glob for full-context ingestion

Supporting changes

• docs/docfx.json — Both generated files added to resource section; llms.md and llms-full-header.md excluded from content build
• .github/workflows/docs-deploy.yml — Runs generation script before DocFX build
• .gitignore — Both docs/llms.txt and docs/llms-full.txt excluded (generated artifacts)

New doc pages are auto-discovered for llms-full.txt via glob. docs/llms.md needs a manual link entry for each new page.

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[github-actions]

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[mattleibow]

@copilot please fix

[Copilot]

@copilot please fix

All three review comments addressed in 01c2e05:

• Header moved out of script into docs/llms-full-header.md
• Hardcoded file list replaced with index.md + docs/*.md glob
• llms.txt now generated from docs/llms.md template, both output files gitignored