Add documentation to integrate middleware

[l-vo]

Introduced in #1472

[greg0ire]

Please comply with the sf docs standards:

Each line should break approximately after the first word that crosses the 72nd character (so most lines end up being 72-78 characters);

[l-vo]

@greg0ire thank you for your careful review. Updated 🙂

[l-vo]

@fabpot @derrabus suggestions applied, thank you 🙂

[greg0ire]

Given where the DBAL discussion is going, I'd use git reflog to revert to a version where Middleware is countable.

[l-vo]

According to the choice in doctrine/dbal#6033, I followed @greg0ire advice and reverted "Middleware is uncountable"