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".
Having the jekyll assets here is a mistake, because they need to be versioned
separately to the spec: we keep old versions of the spec, but they sometimes
need to be restyled to match the latest website.
Make the argument to dump-swagger.py be the name of the output file rather than
the output dir, and give it a sensible default.
Also, use the absolute path to the output file, so that the output path is
relative to the user's cwd rather than the templating dir.
Andrew Morgan