Add a little structure to doc subdirectory

Right now, doc has user-facing and embedder-facing documentation:

• Compatibility.md
• SupportPolicy.md
• TROUBLESHOOTING.md
• Android.md
• FAQ.md
• iOS.md

And long-term developer-facing documentation:

• Architecture.md
• Safelogging.md
• semver_status.md
• Release.md
• testing
• metrics
• ZeroizeStrategy.md

And shorter-term design notes that explain our thinking behind particular aspects of the system:

• BridgeIssues.md
• BridgeAPI.md
• CONFIG-NOTES.md
• Errors.md

IMO we should organize this a bit. I propose this structure:

• doc/ — User-facing and embedder-facing documentation.
• doc/dev/ — Long-term developer-facing documentation
• doc/dev/{testing,metrics}/ — new locations for the current "testing" and "metrics" directories.
• doc/dev/notes/ — Design notes for things that have already been built. These should probably be dated.

I also propose that, where reasonable, we should move stuff from doc/dev/* into our rustdoc.

Bikeshed away!