docs: add SKILL.md and agentic coding related stuff to Contributing

ref: https://github.com/0xPlaygrounds/rig/pull/1384

Review the linked pull request #1384 in the main rig repository to understand what SKILL.md and agentic coding content should be added. Locate the existing Contributing documentation in this docs repository to see where to insert the new sections. The work involves creating or updating markdown files, likely in the root or a docs directory.

索引モデルが issue の本文から書いたものです。