Add usage documentation for Rich Text package

[getdave]

Description

I've been using the @wordpress/rich-text package a fair amount recently. It's super powerful and I really appreciate the work that's gone into this.

That said, to understand the package you must currently learn by either:

• reading the code
• reading the tests
• reviewing current example usages

Whilst it is possible to gain a working understanding of the fundamentals of the package in this way it is not the most efficient method. Given that this package is so critical and underpins much of what Gutenberg can do, this seems like something we should look to address.

This PR attempts this by adding some basic Usage documentation. This covers

• the purpose of the package.
• the fundaments of the RichTextValue object.
• the concept of "selections" (including "collapsed" selections)

I would also like to cover the replacements property of the value object. I would do this but I still don't understand it so I'm going to need to rely on others to add this information.

How has this been tested?

• Read the docs.
• Do they make sense?
• Is any element misleading?
• Is any part inaccurate? - specifically the code / prose-based examples.
• Is my spelling and grammar ok?
• Was there anything missing?

Screenshots

Types of changes

New feature (non-breaking change which adds functionality)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[mkaz]

Thanks for adding the examples, I've already learned more about the RichText package.

I've added a couple of suggestions on rewording in a few spots.

I was a little confused by the Collapsed selection, the first paragraph makes sense. However, the last sentence confused me on what I'm supposed to take away from it.

It looks like you are still working on it since there is a TODO: in the Replacements spot, or was that asking for others to add their input?

This is great so far and thanks so much for documenting and adding examples. It helps!

[mkaz]

Thanks for updating, this is a good improvement to the RichText docs 👍