Install webrick gem for Ruby 3.x compatibility

What does this MR do and why?

In preparation for moving project to Ruby 3.x, follow the guidance at: https://nanoc.app/doc/installation/#ruby-3-x.

We can pre-install this now to minimize the number of moving parts.

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

2. Switch to Ruby 3.0.5 on main branch.

3. Run make setup && make view. Get error:

   INFO: Starting GitLab documentation site...
   
   Captain! We’ve been hit!
   
   LoadError: Couldn't find handler for: puma, thin, falcon, webrick.

4. Switch to Ruby 3.0.5 on MR branch

5. Run make setup && make view. View succeeds.

6. Be on Ruby 2.7.7 on MR branch.

7. Run make setup && make view. View succeeds.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.