Document DirectProductElement

[limakzi]

DirectProductElement and DirectProductElementNC were previously declared in tuples.gd with an empty <Description> block and no GAPDoc label, so they did not appear in the manual at all.

This PR fills in the documentation, component access via indexing and Length, and componentwise arithmetic, followed by two worked examples:

• one showing basic usage with permutations,
• and one showing the operator in the context of a DirectProduct(G, H) with embeddings and projections.

Fixes #3452.

[Copilot]

Pull request overview

This PR adds missing reference manual documentation for creating and working with IsDirectProductElement objects via DirectProductElement / DirectProductElementNC, and ensures the new documentation is included in the reference manual build (fixing #3452).

Changes:

• Add GAPDoc for DirectProductElement / DirectProductElementNC, including indexing/Length, componentwise operations, and worked examples.
• Include the new GAPDoc label in the reference manual section index.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
lib/tuples.gd	Adds a new GAPDoc block documenting DirectProductElement and DirectProductElementNC, with examples.
doc/ref/mapping.xml	Includes the new DirectProductElement label so it appears in the manual.

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[limakzi]

@fingolfin What blocks merge?

[ChrisJefferson]

Hi, I had a look, and this looks good to me. My only question (as you might be best to judge this), is if the tests in DirectProductElement.tst are good enough -- while stuff should always work and be tested, I'm tempted to increase requirements on testing if something is documented (but I do already notice a bunch of tests, and you've got tests in your docs too)