[WIP] Massive documentation overhaul

[xenova]

• replace jsdoc-to-markdown dependency with custom parser for finer control
• fix broken links
• improve + update docs
• upgrade build process
• add agents.md and skills

opening PR early to get early feedback and to get docs previews

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.