Docs: add documentation for WordPress.DB.SlowDBQuery

[petitphp]

This PR add documentation for the WordPress.DB.SlowDBQuery sniff.

Part of #1722

[petitphp]

Posted a message in the core-performance slack channel to :

• clarify why the tax_query is considered a potential slow query and
• if a performance handbook with specific info about slow queries is available

[jrfnl]

@petitphp 👋 Just checking - did you get any useful responses in Slack ?

Also: I noticed this PR is still in draft. Do you want me to review already ? Or are you still working on it ?

[petitphp]

@jrfnl Sorry for the delayed response ! This PR is still a WIP and doesn't need to be reviewed for now.

I'm still working on this, I had great responses in slack with some context on why this rule was created.

I'll try to finish the doc and mark the PR as ready for you to review.

[jrfnl]

@petitphp Thanks for letting me know!

[rodrigoprimo]

@petitphp, I was just wondering if you'll have a chance to finish this off in the near future. It would be great if this PR could be included in the next WPCS release.

If you haven't got time or lost interest, please let us know and we'll see if we can find someone to take over. Thanks!

[petitphp]

@rodrigoprimo Thanks for the ping, I didn't have the time to work on this for some time.

I'll see to finish this PR. I'll mark it as ready for review as soon as i'm done !