Content related bugs should be reported on the forum, so they can be discussed and so that people notice the changes. It's a means of broad peer review and documentation.
See above, the same applies to pull requests that aren't structural or fixing simple typos.
It's all here, developed in the master branch and then automatically generated into gh-pages via Travis (since we use a custom plugin and want a downloadable zip of the result).
For anything that could need wider consensus, present the idea on the forum first.
Try not to introduce spelling and grammar bugs.
If possible, when changing opcode/action docs, check any other versions of the opcode. Perhaps the change applies there too.
To make file format info machine readable, check this: Gibberlings3#84 (comment)
Action format: Gibberlings3#84 (comment)