add stack pack guidance into each LH audit guide

[paulirish]

Fixes #3674

Changes proposed in this pull request:

• Add each LH audit's stack pack content into the documentation.

Built via https://github.com/GoogleChrome/lighthouse/compare/updatewebdevwstackpack

[github-actions]

Alex Recommends Report

Alex recommends the following language changes, but Alex is a regular expression based algorithm, so take them with a grain of salt.

src/site/content/en/lighthouse-performance/user-timings/index.md

Level	Location	Word	Recommendation
⚠️	43:6	just	just may be insensitive, try not to use it

src/site/content/en/lighthouse-performance/uses-rel-preconnect/index.md

Level	Location	Word	Recommendation
⚠️	41:47	simple	simple may be insensitive, try not to use it

src/site/content/en/lighthouse-performance/uses-responsive-images/index.md

Level	Location	Word	Recommendation
⚠️	34:27	just	just may be insensitive, try not to use it

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[kaycebasques]

Hellooooo just noting that this is on my radar and I'll get to it next week (fingers crossed)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. To prevent this from happening, leave a comment.

[paulirish]

not stale

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. To prevent this from happening, leave a comment.

[stale]

Closing this issue because it has been marked as stale for more than 7 days.

[kaycebasques]

I need to make various edits to the writing (breaking lines at 100 characters, etc.) and I don't think I can push to your fork so I will close this and continue in a new branch.