Add documentation section on reproducible builds

Pydoctor support reproducible builds, a documentation explaining this feature is currently missing.

See work in #84, #99, #146 and #488.

There is still some ongoing discussion in #521 and i believe #138 can be closed as completed with a couple of tests...

- For instance generating documentation with macos, linux and windows and asserting that the output hash is the same for the three builds.