docs: add portal and profile API endpoints

[cnaples79]

Summary

• add dedicated API section under Authenticate docs
• document JSON portal endpoints (login, whoami, beacon)
• document administrative profile/server API endpoints with examples

Rationale

• issue #459 requests documenting available API endpoints

Changes

• docs/authenticate/api/category.json: new API subsection
• docs/authenticate/api/portal.md: portal JSON endpoints
• docs/authenticate/api/profile.md: admin/profile API endpoints

Fixes greenpau/caddy-security#459

[greenpau]

@cnaples79 , thank you for the PR! 👍

After reading it, I added f8c6d93 to provide you with some framework to work with.

When reading, the sense that the content is AI output did not escape me. It is very mechanical and tries providing OpenAPI type of spec in markdown format.

When you use AI, try to prompt it in such a way that you focus on hierarchies, explanations as to why and what, and validating what is actually listed. I gave some examples in "Server API".

For example, "Portal JSON Endpoints" in documentation does not make sense. In fact, serving it from /json endpoint does not make sense (which I am going to change soon). Rather "Portal API" might be better for understanding.

Please amend your PR and start with one section, e.g. "Portal API". After that ping me here and I will review it.