Consolidate documentation about documentation into docs directory

[jimwins]

This pulls info that was on doc.php.net and in other places within this repo into one docs directory so everything is in one place.

The plan is to have it be published on http://doc.php.net somewhere, too (and redirect old /tutorial links as necessary), but it's browsable through GitHub, like https://github.com/jimwins/doc-base/blob/clean-up-phpdoc-docs/docs/README.md

[jimwins]

I ripped out all of the out-of-date info on how the manual is built and just direct questions to systems@php.net for now. We can add back info about where the scripts that do the build live, but it looks like not quite all of them are in the systems repo yet.

[haszi]

As someone who started to contribute to the documentation less than a year ago, I can say that this kind of consolidation and clarification on how to contribute is badly needed. Thanks for this!