Simplify and update the content of the handbook

[rianrietveld]

This PR reorganises the content of the handbook.

Related issue: #1
Preview: https://github.com/wpaccessibility/accessibility-team-handbook/blob/9c8c49cce0bf948821661b4ec75fe0c86f573ee1/index.md

Main goal was to simplify and update the content.

We keep 3 main focus points:

• The accessibility of WordPress
• The accessibility-ready program
• Documentation and education

I tried to keep those main focus points reflecting in the rest of the content. The working groups are gone, and replaced by the focus points.

The most popular pages in the section standards and best practices now have placeholder text referring to the knowledge base and if available to the main resources on wp.org.

The content for standards and best practices and the accessibility-ready guidelines now lives in the WP Accessibility Knowledge Base.

I also added the content of the document about the Accessibility-ready theme testing instructions and reworked it to make it usable for all contributor days.
This Accessibility-ready theme testing instructions page needs review bij @amberhinds and @joedolson as I'm no expert in this.

The plan is to move this repo to the WordPress organisation when approved, replacing the current handbook.

[amberhinds]

@rianrietveld RE the Accessibility-ready theme testing instructions page, I don't think this should exist.

The tasks and the sheet described are only relevant for the next few months. After the initial run through of themes, none of this will be relevant. Also, this replicates instructions already provided in https://wpaccessibility.org/docs/accessibility-ready/testing-themes/

[rianrietveld]

@amberhinds I removed the Accessibility-ready theme testing instructions page and added links to how to test and to the requiremenst in the Knowledge Base.

[joedolson]

While streamlining the team focuses is great, I think we should have some information to let people know how to contact us to request review on additional features, such as Playground, TV, Meta, etc. Even though those areas aren't our primary focuses, we're available to review changes and provide feedback.