Feat: Brotli support

[lena-larionova]

Description

We added Brotli compression support in 3.6 but it was never documented. Adding a short doc on how to enable it.

Notes:

• I considered adding it to the Nginx directives doc, but there's nothing in there about configuring specific groups of directives for a goal - that topic is a more general thing.
• I'm also not sure about putting this under "reference", but honestly, we don't really have a place for content like this at the moment.

Addresses https://konghq.atlassian.net/browse/DOCU-3911.

Testing instructions

Preview link: https://deploy-preview-7692--kongdocs.netlify.app/gateway/latest/reference/brotli/

Checklist

• [x] Review label added
• [x] Conditional version tags added, if applicable.

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