chore(docs): Better explanation on how to allow tools in external directories

[OpeOginni]

What does this PR do?

Gives the external_directory rule in permissions its own section in the docs to better help users know how to make use of this feature, and also stay safe using it, by talking about what happens when a directory is added here, letting them know they can further restrict these directories from other tools like edit, but keeping read.

Closes#10848

How did you verify your code works?

Ran the web package and checked out the docs.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, here are the potentially related PRs:

Related PR:

• feat: add granular external_directory permission configuration #5905 - feat: add granular external_directory permission configuration - This PR adds the actual external_directory permission feature. Your PR (10862) appears to be documentation/educational content about how to use this feature properly and safely.

Other Related PRs:

• docs: add Autonomous Mode section to permissions docs #6968 - docs: add Autonomous Mode section to permissions docs - Related to permissions documentation
• feat: Permissions dialog #8859 - feat: Permissions dialog - UI feature for permissions

Your PR (10862) is documentation-focused to help users understand the external_directory permission feature that was introduced in #5905, rather than a direct duplicate.