Fatheads: Update documentation for cover/ dir filenames, testing and PR autodeployment

[moollaza]

Description

• Updated testing documentation to mention duckpan test approach
• Add details about special cover/ dir filenames, and the redirects they automatically generate
• Describes new redirects.txt file for simplified redirect creation
• Describes new trigger_words.txt file and format
• More details about Fathead auto-deployment

People to Notify

@tagawa @jbarrett

[tagawa]

Oh, and we should make sure the example jQuery keyword redirect works before merging this (it doesn't currently).

[tagawa]

jQuery keyword redirect seems to be fixed now, e.g. https://duckduckgo.com/?q=jquery+show%28%29+function

[moollaza]

This PR is now ready for review 👍

[MariagraziaAlastra]

LGTM 👍