doc/contribute.html: clean up HTML and formatting

Mostly just formatting and minor cleanup:

- regularize HTML (add </p> etc.)
- remove all errors caught by tidy
- start all sentences on new line for easy editing

Some wording changes, but there will be more to come.
It seemed there were already enough edits to send it out.

Update #24487

Change-Id: I613ce206b1e8e3e522ecb0bbcd2acb11c4ff5bae
Reviewed-on: https://go-review.googlesource.com/113015Reviewed-by: Ian Lance Taylor <iant@golang.org>