Switch Pricing page to Themes, switch to Themes from Capabilities

Why is this change being made?

This MR builds on top of MR (!82555 (merged)) which adds theme field to features.yml, and makes multiple changes to high traffic pages:

• Switches from Capabilities to Themes. Capabilities no longer exist!
  • Tries to update the Pricing handbook page. Adds mermaid chart, switches to themes from capabilities.
• Switches the pricing page from relying entirely on pricing_experiment.yml to using features.yml and pricing_themes.yml
  • NOTE: This means that some "features" that were on pricing page are no longer. Specifically the "X minutes more than other tier" and the "24/7 Support" one. We can consider adding these back as Themes if we want.
• Updates /features/by-capability/ to /features/by-theme/ - and drives the page off pricing_themes.yml and features.yml
  • Also adds an outlier area. We can decide if this is useful or not.

Pages to check

• Pricing page
  • Original - https://about.gitlab.com/pricing/
  • New - https://jl-add-themes.about.gitlab-review.app/pricing/
• Pricing by Themes
  • Original - https://about.gitlab.com/features/by-capability/
  • New - https://jl-add-themes.about.gitlab-review.app/features/by-theme/
• Pricing handbook page
  • Original - https://about.gitlab.com/company/pricing/
  • New - https://jl-add-themes.about.gitlab-review.app/company/pricing/index.html

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)