Draft: Consider using stable URL tag, consider removing unsupported versions

[ericglau]

To improve SEO and clarify version usage, this draft PR applies the following ideas for consideration/discussion:

• If the user is on the current version, changes the version in the URL to be current. This will allow the current versioned URL to always point to the current version.
  • I haven't seen the current tag used in other documentation sites, but our docs UI describes versions as "Current version".
  • We could consider latest or stable as these are more common, but:
    • latest is confusing if there is a newer prerelease than the latest stable version e.g. 2.0.0-alpha.0 vs 1.0.0
    • stable works but in the case of 5.x, 4.x, 3.x, they are technically all stable.
    • Should we consider a combination of latest and stable (with stable having priority)?
• To provide more context due to the URL change above, always show the version number in the left navigation, even if there is only one version, unless the product is unversioned.
  • For example, Upgrades Plugin docs are unversioned, while Uniswap Hooks has one version.
• Completely removes documentation for versions of products that are no longer supported, since having many old versions (if no one is/should be using the old versions) could be confusing.
  • Decide if docs for unsupported versions are still useful
  • Review which versions should be supported for each product
  • Note: For entire products (rather than specific versions) that are deprecated, they are not removed but just unlisted in the navigation as before. Information in those are still sometimes useful, for example information on how to migrate or conceptual information on topics.

Related to #428