chore(config): add instructions and skills for documentation generation

[etiennej70]

PR Description

What this PR Provides

• Add copilot instructions for the documentation generation (markdown files)
• Add skills for ADR generation and Documentation site content generation

Fixes

-N/A

Review

The reviewer must double-check these points:

• The reviewer has tested the feature
• The reviewer has reviewed the implementation of the feature

How to test

• Checkout the branch
• In copilot chat, try /create-architectural-decision-record or /update-documentation-site to trigger the skills specifically
• See also the general instructions file added automatically when any agent session is started

Breaking changes (if any)

• N/A

[CLAassistant]

All committers have signed the CLA.