docs: update styling and add beta/legacy docs

[tylandavis]

• Updates various components within the docs and elsewhere for improved readability and consistency.
• Adds ability to fetch a beta and legacy version of the docs.
  • Set the environment variable NEXT_PUBLIC_ENABLE_BETA_DOCS to true to enable the beta docs.
  • Specify a branch, commit, or tag with NEXT_PUBLIC_BETA_DOCS_REF. The default for the beta docs is beta.
  • Set the environment variable NEXT_PUBLIC_ENABLE_LEGACY_DOCS to true to enable the legacy docs.
  • Specify a branch, commit, or tag with NEXT_PUBLIC_LEGACY_DOCS_REF. The default for the legacy docs is null, and will fallback to the main branch.