guide: Extract `templating` and `foreach` features from `Pipeline Files (dvc.yaml)`

[daavoo]

The templating and foreach features are documented as sections under user-guide/project-structure/pipelines-files

I believe these features are relevant enough to have a separate section (not sure where) and the current page should be reserved for defining the parts of dvc.yaml.

We should improve visibility, revisit the narrative and extend the information and examples for these features.

See also #2883

[shcheklein]

Related to @iesahin 's work on #3414

[shcheklein]

@daavoo @dberenbaum could you please review and let us know your thoughts in the #3414 ?

[daavoo]

Related to @iesahin 's work on #3414

Not sure I follow. I don't see any content regarding templating or foreach in the P.R.

[jorgeorpinel]

I think that's the point: it should be in that PR or another one to address the underlying issue (#2883).

[dberenbaum]

Now also needs matrix and hydra composition. Looks related to #4670 and #2313. Should we consolidate into this issue?

TBH I'm not sure we can take it now, but I would consider it a "high p2."

[dberenbaum]

Also related to #3896