Reduces friction for CUJ "I want to know about the supported manifest keys"

[AmySteam]

Fixes #4254. #163. #1774. #1859

Changes proposed in this pull request:

• Remove keys for internal use only
• Add links to missing docs (PR#4293, PR#4289, Updates old homepage_url key documentation #4291, [extension-docs] Add manifest key pages: author, automation, CSP, input_component #3837)
• Add code samples to all manifest key pages
• Standardize format for all keys: "key name and link": "key type"
  • For example: "action": {...},
• Add manifest code samples from the Getting Started guide tutorials so that developers can copy/paste them as examples in their projects.

Keys ready for review

• manifest_version
• sandbox
• version
• description
• externally_connectable
• default_locale/
• key
• optional host permissions

Staged links
Manifest keys

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[jpmedley]

@AmySteam Did you find anything in here that should be mentioned in the upgrade materials?

[AmySteam]

@jpmedley Not really. Mostly cleaned up broken links and removed mentions of Chrome apps.

[jpmedley]

@AmySteam, if this is ready for fact checking, please assign it to @dotproto.

Thanks

[AmySteam]

Hey @dotproto, this is ready for a technical review; I assigned it to you 🙂