This generates templated files using some parallelism, reducing the
total mksite time to less than half in my tests. Increasing parallelism
even further is possible, but would make the code harder to understand.
The obvious technique of generating each templated file in its own
process is actually far slower because the overhead of process creation
dwarfs the time spent processing the template, which is on average very
small and quick.

The JSON schema is simple and compatible with the one published in the
Go Vulnerability Database. Security advisories and bugfix advisories
each have their own index.

Open Source Vulnerability format is a standard for publishing
vulnerabilities in Open Source projects and is defined at
https://ossf.github.io/osv-schema/

While the old indentation works, the new one passes yamllint, making it
easier to automatically check for problems.