Make sure brotli is available when needed

What does this MR do and why?

With !1042 (merged), we started using brotli instead of gzip for compression.

However, brotli isn't shipped in our Docker image for site builds:

• Example from MR !1042 (merged): https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/10860247555
• Example from MR !828 (merged): https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/11054413185
• Local confirmation:

  $ docker run --rm -ti --platform linux/amd64 registry.gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/docs-gitlab-com-builder:hugo-0.145.0 sh
  /src # brotli
  sh: brotli: not found

In the short term, we can install it at runtime. In the longer term, we can ship it in our builder images.

Job with brotli available: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/11056126571.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.