Adds docs to nginx and refactors both Httpd and Nginx benchmarks.
Key changes:
- Add docs and make nginx tests the same as httpd (reverse, all docs, etc.).
- Make requests scale on c * b.N -> a request per thread. This works well
with both --test.benchtime=10m (do a run that lasts at least 10m) and
--test.benchtime=10x (do b.N = 10).
-- Remove a doc from both tests (1000Kb) as 1024Kb exists.
PiperOrigin-RevId: 329751091
b9b6660dc4