Add latest flag to docs_config.js

[pdavies]

Hello there!

It's a common problem to find old docs surfacing at the top of search results - see for example https://x.com/jskalc/status/1816920900660236370. Maybe there are tricks that would improve the search rankings, but as a starting point, I thought it would be useful to warn users when they are looking at old versions, which some languages' docs systems already do.

The main PR for that is https://github.com/elixir-lang/ex_doc/pull/1918 - but it needs the docs host, i.e. hexdocs, to flag the latest version in docs_config.js. This PR does that.