Added globals section
Added a note on caching
Documented custom engines and DelegatingEngine
Applied comment from @weaverryan
feature #3589 Finishing the Templating component docs (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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Finishing the Templating component docs

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | all
| Fixed tickets | #955

Fixed the remeaning tasks of https://github.com/symfony/symfony-docs/issues/955#issuecomment-4566941

Commits
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b215395 Applied comment from @weaverryan
9f34675 Documented custom engines and DelegatingEngine
e243485 Added a note on caching
3a9f140 Added globals section