added a sentence to the ref manual subsection on direct sums of algebras

[cdwensley]

This PR just adds the following sentence to section 62.9.14 of the reference manual: "Experimental methods for Embedding and Projection for direct sums of algebras are included in version 1.26 of package XModAlg."
The purpose of this PR is really to get some feedback.
The XModAlg package may well not be the best place for these methods, and the 3 obvious options are:
(1) to leave these developments in the XModAlg package;
(2) to move them to the Utils package;
(3) to move them into the main library, where other adjustments could then be made.
They have been added to XModAlg to be used when constructing actions on direct sums of algebras and direct sums of crossed modules of algebras. The full set of declarations/methods not specific to XModAlg is [ DirectSumOfAlgebrasInfo, Embedding, Projection, DirectSumOfAlgebraHomomorphisms ].
Advice, suggestions, etc. most welcome!

[ChrisJefferson]

My first thought is, what makes the methods experimental?

I feel in the manual we should only suggest solid, well-tested methods we expect to hang around for a long time.

This is seperate from where they should live. Also, I think we generally don't advertise packages in the manual, I'm not sure where we would stop if we did.

[cdwensley]

'Experimental' just means that they are not fully tested. The reason for mentioning the methods here is that users would expect them to be available, in line with other products. Indeed, the code in algebra.gi contains the comment "#T embeddings/projections should be provided!".

[fingolfin]

Thank you @cdwensley and sorry for neglecting this PR for so long. Teaching is finally over, and I'll soon be gone for two weeks of vacation but I hope to be able to get more on top of things again starting mid august.

Regarding this PR, I would prefer to just add these methods to the library. Would you be open to make a PR in this direction?