feat(docs): add comprehensive README for documentation directory

[SashaMalysehko]

What ❔

This PR adds a comprehensive README.md file to the docs directory that:

• Explains the documentation structure and organization
• Lists key documentation files and their purposes
• Provides clear guidelines for documentation contributors
• Includes information about building docs locally
• Links to the official zkSync Era documentation

Why ❔

A well-structured README in the docs directory is essential because it:

• Helps new contributors quickly understand the documentation structure
• Establishes clear guidelines for documentation contributions
• Makes it easier to maintain consistent documentation standards
• Improves the overall developer experience by providing clear navigation
• Ensures documentation remains organized as the project grows

Checklist

• PR title corresponds to the body of PR (we generate changelog entries from PRs).
• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• Code has been formatted via zkstack dev fmt and zkstack dev lint.