Increase font size for tabbed content

What does this MR do and why?

Increases font size for tabbed content. GitLab UI sets this as 0.875rem, but on the Docs site, our regular text content is 1rem. The sizing should match between the tabs and other content.

Screenshots, screen recordings, or links to review app

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Compile and view the site: make view
3. Look at a page with tabs (e.g, this one or this one) and verify the font size matches the rest of the page content.

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.

Closes #1374 (closed)