Doc Revision: OAuth

[markstur]

Summary of Requested Changes:
There needs to be more OAuth docs.

I tried to follow the example that is part of MCP Integration but ran into problems. Even if it worked, that is the wrong place to introduce OAuth.

Requirements
Enter the requirements that would warrant success for this issue.

Task Checklist

• Contributing Guideline Compliance: I read and complied by "Documentation Contribution Guidelines" section of the Agent Stack project CONTRIBUTING.md guide.
• Snippet Test: Verified all code works in a fresh environment (fresh agent stack install and clean environment).
• Visual Preview: Checked formatting/layout via Mintlify.
• CLI Sync: Ran mise run agentstack-cli:docs (if applicable).
• Location: Updates placed in docs/development for go live during next release or both docs/development and docs/stable for urgent go live.

[markstur]

See bug: #2067 regarding a possible bug in the MCP example, but bigger picture docs belong somewhere else.