Implement feature flags list page

Port generate_feature_flags from build_site.rake.

This script creates the content for this page: https://docs.gitlab.com/ee/user/feature_flags.html

For a Hugo version, we'd probably need to do something like:

1. Extract the feature flags from gitlab, like we do in Ruby here: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/lib/tasks/build_site.rake#L67

2. Combine all of these individual YAML files into a single YAML file.

   • The Nanoc one looks like this after its built: https://gitlab.com/-/snippets/3739917.
   • We can ignore this file in Git and Yamllint since it's machine-generated.
   • To work this in with our existing build process, it'd probably work well to do this just like we did for building the breadcrumbs.json file in !177 (merged):
     • Add a new file (and test file) in cmd/gldocs/tasks, like build_feature_flags.go
     • Call whatever function it provides from cmd/build.go

3. Create a Hugo template to turn that YAML into a table.

• Nanoc version: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/layouts/feature_flags_table.md.erb

It might make sense to just read the individual YAML files directly from the Hugo template (so, skipping step 2, or having the script just move the YAML files into data/ for Hugo). I'm not sure why it was necessary to combine them all into a single YAML file.