[docs] Custom template clarifications and other minor enhancements

[jimschubert]

Updates CLI helpers to output docs pages and docs/generators.md so it may allow navigation on GitHub. This reintroduces work from #4120. The version of docusaurus we were on didn't index nested files correctly, so generators/ada.md wouldn't create a link to /docs/generators/ada. Part of this fix was to remove the id attribute from these files, which may increase the likelihood of id collisions in the future. I think this is acceptable if it makes the docs more easily discoverable and browsable on GitHub.

This also adds some clarification to templating.md and cutomizations.md based on some discussion in Slack over the past week. I reviewed these two documents and the confusion about whether users needed to modify mustache templates only vs creating a new custom generator was apparent after the discussions in chat. Hopefully the brief overview in templating.md and the expansion in customizations.md will reduce any further confusion.

This PR also updates docusaurus and incorporates two new features: collapsible sidebar and scroll-to-top.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before .
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @OpenAPITools/generator-core-team @wing328
cc @vitorvezani (fyi)

[wing328]

LGTM