Include READMEs in main framework pages of the API documentation

[p8]

Currently when opening the main framework API pages there is no introduction to the framework. Instead we only see a whole lot of modules and the gem_version and version methods.

By including the READMEs using the :include: directive each frameworks has a nice introduction.
For markdown READMEs we need to add the :markup: directive.

[ci-skip]

Thanks to @zzak for the tip of the :include: directive.

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[p8]

I've also created a PR for sdoc to move the file links to the bottom.
This would result in the following:

[zzak]

It would be nice to get a preview of this on the PR, I'm actively trying to make that happen. 🙏

In the mean time I will have to pull this down and take a look.

Too bad the RDoc autolinks Model from the readme, we will have to escape it (e.g. Active \Model) like we do with RDOC_MAIN.md 😢

[zzak]

By including these there is a side-effect that the methods defined in those files are now considered public API if they are not otherwise :nodoc:'d. That may have been intentional, however. Just pointing it out.

[p8]

I'll :nodoc: those.

[p8]

@zzak If any of those methods are considered private we should :nodoc: them.
Other generators like Rdoc Info do include these files already and will consider it public API:
https://www.rubydoc.info/github/rails/rails/ActiveSupport.cache_format_version=

For some methods we already add :nodoc:, so I'm guessing it's not common knowledge some of these methods aren't shown in the API docs :

rails/activesupport/lib/active_support.rb

Lines 97 to 101 in 80c51c1

	cattr_accessor :test_order # :nodoc:
	cattr_accessor :test_parallelization_threshold, default: 50 # :nodoc:
	@error_reporter = ActiveSupport::ErrorReporter.new
	singleton_class.attr_accessor :error_reporter # :nodoc:

So I guess this fixes a bug where public API methods weren't shown before.

[zzak]

My really honest guess is that API being visible on the website is secondary to the :nodoc: it might have in the source code in terms of weight it has over whether or not it's intended to be public. This is a fair assumption IMO.

If there is no :nodoc: it should be considered public otherwise. After that it's either find the person with context or can make a decision about the visibility or take a wild guess 😂

[zzak]

This LGTM, thanks @p8!

[p8]

Thanks @zzak!