migrate frontend to docusaurus (#9014)

* delete loads of really important stuff that we definitely need

* v basic MVP smoosh docusaurus PoC into repo

* TODO

* delete more really important stuff

* TODO

* tidyup: use run-s

* don't redirect images used in frontend to raster proxy

* fix routing

* preserve the /endpoint link

* delete the blog (for now)

I would quite like to re-add this at some point
but its not really the top priority thing right now

* content edits

* appease the lint gods

* update danger rules

* remove placeholder

* cypress tests

* dockerhub --> ghcr

* Revert "dockerhub --> ghcr"

This reverts commit ef74cbb26b1c24ce987a8975e60313682f9161f8.

* downgrade lockfile format

* implement defs/BASE_URL

* fix e2e build

* actually fix cypress tests

* always run cypress tests on build

* this never worked

* add command for docusaurus:clear

* delete more code we don't need any more

* update ESLint/prettier config

* delete unsused exports

* documentation updates

* delete a fairly large chunk of our dependency tree

* allow base_url as build arg to Dockerfile

* fixup dockerfile

* work out base url at runtime if not set

doing this at image build time is not the right approach

* remove gatsby monorepo from closebot

* rename HomepageFeatures to homepage-features