Docs: Add Skill Discovery Across Docs, READMEs, and `llms.txt`

[mihar-22]

Context

The official skill cannot help developers unless it is visible at the installation, repository, package, and agent-documentation entry points.

Outcome

Add the skill to installation pages, Build with AI, the root README, package READMEs, and llms.txt headers.

References

• docs(site): document the Video.js agent skill #2888
• feat: add videojs agent skill skills#1

[github-actions]

This appears to duplicate the skill-discovery work already tracked in #2894: its scope explicitly includes discovery across docs and READMEs, and PR #2888 implements these same installation, Build with AI, README, and llms.txt surfaces. Recommend tracking this scope under #2894 (already on Roadmap project #7 with the GA milestone) rather than adding a separate roadmap item; #2888 and videojs/skills#1 are the relevant delivery dependencies. P1 is appropriate at the umbrella level because this gates the GA agent experience.