Move docs for client-side secret detection to its own page

Documentation of client-side secret detection is almost buried/hidden at the end of the pipeline secret detection docs.

This is a problem because:

Including it in the pipeline SD page implies it's part of that feature, yet the docs state it is not. This is confusing.
Including it in a section titled "Warning" implies there is some risk involved, and so the user needs to be warned of it. That's absolutely the opposite of what we want to communicate.
The docs state "Issue 405147 tracks efforts to align these two types of protection.", referring to client-side and pipeline secret detection. This is incorrect. Each of these features will expand in scope, but they are not being "aligned".

Create a new docs page, a child of the main secret detection.
- Move the client-side secret detection docs content from the pipeline secret detection page (as noted above) to this new page.
Edit the list of SD methods/features in the main secret detection page, so that instead if saying "two methods", it lists the three available methods and describes briefly the intent of each.

Anyone.