feat: Add support for GitLab-flavoured markdown note blocks to astro

Jack Stephensonrequested to merge
astro-add-support-for-gitlab-note-blocks into master

This change follows one for our Cloudflare Terraform Module docs site, which added support for GitLab-flavoured note blocks.

This is implemented as a remark plugin that transforms the blocks AST into the Aside-based AST for rendering as we want.

Related cloudflare docs MR: https://gitlab.com/gitlab-com/gl-infra/terraform-modules/cloudflare/docs/-/merge_requests/6

This allows us to continue using our existing note blocks syntax whilst getting the benefits of the Astro aside blocks.

Also included in this change is a revert of the change to use Astro aside blocks, to revert to using the GitLab flavoured note blocks that are now supported.

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