[Docs]: Liquid 4 docs hard to find / Liquid version is unclear

Motivation

The official Liquid documentation (as linked in Jekyll's doc) refers to its version 5.0. However Jekyll uses Liquid 4.0. It makes it hard to understand what is valid or not (e.g Liquid 5 says "The include tag is deprecated, use render").

Also, Jekyll doc doesn't explicitly say that it still uses Liquid 4.0, I had to look up the gemspec from the source.

Suggestion

Different suggestions :

• Update Jekyll to use Liquid 5, and updates the Jekyll docs accordingly
• Fork Liquid 4 docs and embed them into Jekyll doc
• At least, put a warning in Jekyll doc regarding the version mismatch issue