fix(docs): brotli is required on all platforms

[gma]

Changes

The previous fix in #3870 suggested that brotli was only required on ARM. It's called in ./bin/start so we need it on all architectures.

I've added made a new paragraph for brotli, after the platform specific pip instructions.

Checklist

• [x] Titles are in sentence case
• [x] Feature names are in title case
• [x] Words are spelled using American English
• [x] I have checked out our style guide

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