Document that we avoid changing src/ext
We only change src/ext when necessary for integration with tor, or portability across platforms.
There was some confusion about this in legacy/trac#32626 (moved), so I'd like to update one or more of the following documents:
- src/mainpage.md
- src/ext/README
- doc/HACKING/CodeStructure.md
We might also want to change our merge policy, but that's a longer process.