Update docs layout and content

[jeffpaul]

Is your enhancement related to a problem? Please describe.

• Update existing Docs section of hookdocs site to Tutorials
• Add new Docs section, with WP-CLI Commands as initial sub-selection there that cuts the WP-CLI Commands bit from the docs home page into a new page
• Add new Headless Integration to Docs section with content as noted in Headless integration docs #8
• Add new Post Content Type to Docs with content from https://github.com/globeandmail/sophi-for-wordpress#post-content-type (then remove that content from both readmes)
• Add new Canonical URL to Docs with content from https://github.com/globeandmail/sophi-for-wordpress#canonical-url (then remove that content from both readmes)
• Add new Object Caching to Docs with content from https://github.com/globeandmail/sophi-for-wordpress#object-caching (then remove that content from both readmes)
• Add new Sophi API Empty Response with content from https://github.com/globeandmail/sophi-for-wordpress#sophi-api-empty-response (then remove that content from both readmes)
• Rename "Documentation" to Developer Documentation
• Update docs site title from Welcome to the Sophi for WordPress Plugin Developer Documentation to Sophi for WordPress Developer Documentation

Designs

n/a

Describe alternatives you've considered

Keep as-is, but I think the sum total of the changes above make the dev docs more centralized to the GitHub Pages site.

Code of Conduct

• I agree to follow this project's Code of Conduct

[jeffpaul]

@iamdharmesh would be helpful to get these changes made going into the large 1.3.0 release for the Sophi Overrides and further tidy up our dev docs for Sophi.