This PR does two things:
* Updates the git submodule for the new spec hugo theme ([google/docsy](https://github.com/google/docsy)) to our fork ([matrix-org/docsy](https://github.com/matrix-org/docsy)) which has a couple changes to load JS from local sources instead of remote, as well as allowing sites to override the URL that font CSS files are loaded from. Note that my definition of "font CSS" files here as CSS files that contain settings and point to locations of where font files (`.woff2`) are located.
* Adds a script (and the files generated as a result of running that script) that can take a google fonts URL, download the fonts it points to and spit out those fonts as well as a font CSS file pointing to them for local distribution. We then use the resulting font CSS file in our project variables.
This brings the benefit of not serving files from a CDN which can track users across the web, as well as inadvertently pinning docsy to a specific commit. The downside is that we need to remember to update [matrix-org/docsy](https://github.com/matrix-org/docsy) when needed (and apply the patches on top, though they're quite small).
Since we're deploying to https://spec.matrix.org/unstable now. This only updates the `baseURL` for our buildkite build. The default is still `/` when doing a local build.
This PR attempts to update the CI of matrix-doc to build [the new spec redesign](https://github.com/matrix-org/matrix-doc/issues/2906). It does so by additionally building the new spec in parallel to the old. The plan is to continue to host the old spec at https://matrix.org/docs/spec, while the new spec will be at https://spec.matrix.org. Eventually we will retire the old version of the spec, and have the old URL redirect to the new one.
In detail, this PR:
* Adds a new step to CircleCI to build the new spec with `hugo`. This step uses alpine, grabs some dependencies, and then builds the HTML.
* We needed to hand some specific options to hugo for CircleCI in order to continue allowing CircleCI to host temporary builds of the spec after each CI run. This required changing some assumptions related to relative paths.
* CircleCI's artifacts hosting is also quite limited. Specifically it will not automatically resolve `/some/path` to `/some/path/index.html`, which our hugo theme relied on. Fixes were implemented for this, but we may want to consider switching away from CircleCI artifacts as a host, and using something like [netlify](https://www.netlify.com/) instead.
* Modifies the existing Buildkite pipeline step to build both the new spec in a separate step. It additionally modifies the old spec to be built with alpine. (Separate out into another PR)
* We'd like to separate out the deployment of matrix.org from the new spec. Therefore a new step, with a separate artifact build (`spec.tar.gz`). We will eventually remove the old step and the matrix.org build trigger.
* Modifies `pyproject.toml` to update the config of [giles](https://github.com/OpenAstronomy/baldrick/blob/master/baldrick/plugins/circleci_artifacts.py), which is what creates the "docs", "swagger" links in the CI steps for matrix-docs PRs.
* A new step was added for the new spec. The old spec was renamed to "legacy".
Thibault Martin