Document full extensions directory and extensions in armbian/os and armbian/community repositories

[coderabbitai]

Context

This is a follow-up from PR #851 which adds a FAQ section to the Extensions documentation.

Request

@EvilOlaf suggested:

Perhaps mention the whole directory as well as those in armbian/os and/or armbian/community repository.

Task

Update the Extensions documentation to mention:

• The whole extensions directory (not just individual examples)
• Extensions available in the armbian/os repository
• Extensions available in the armbian/community repository

References

• Original PR: Extensions: add FAQ section and make entry for notable extensions #851
• Comment thread: Extensions: add FAQ section and make entry for notable extensions #851
• Requested by: @leggewie
• Suggested by: @EvilOlaf