RNMobile: Add integration test guide

[guarani]

Description

Adds a guide to integration tests on Gutenberg Mobile.

How has this been tested?

Read through the guide to ensure legibility and accuracy.

Types of changes

• Updated docs

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).

[fluiddot]

I reviewed the document and it looks great to me 🎊, I only added a couple of comments that are minor, thanks @guarani for adding the guide 🙇 !

[guarani]

@fluiddot, I've changed the document title 01b9e87, change the heading sizes and added a table of contents 6d90edd, and made the screenshot of the Accessibility Inspector a little wider for better readability 9917920.
It's now ready for a re-review.

[fluiddot]

LGTM 🎊 !

@fluiddot, I've changed the document title 01b9e87, change the heading sizes and added a table of contents 6d90edd, and made the screenshot of the Accessibility Inspector a little wider for better readability 9917920.
It's now ready for a re-review.

Thanks for the changes, I really liked the idea of the table of contents 💯 .

[guarani]

Thanks for all the work that went into the documentation! 👍