Expose and extend the thematic tutorial on symmetric functions

[nthiery]

Sage needs a good thematic tutorial on symmetric functions. An embryo is in sage.combinat.sf.symmetric_functions, but it's quite indigent (I wrote it, and take the blame):

• Reference it from doc/en/reference/combinat/symmetric_functions.rst and highlight it as the *main* entry point for symmetric functions
• Make sure sage.combinat.sf? points to this tutorial
• Extend it! (for general use, but also for kschur and the like)

References:

• ​MuPAD-Combinat's tutorial on Symmetric functions (​Sources)

• ​Francois's advanced tutorial for MuPAD-Combinat

[jbandlow]

I've put a first version on the sage-combinat queue. It can be built with sage -docbuild thematic_tutorials html. Currently no effort has been made to coordinate with #8533, which will have to be done eventually. I will continue to work on this, but suggestions and advice are welcome here.

[nthiery]

This ticket can be closed as soon as #14900 is merged. #5457 took care of providing a nice tutorial, and #14090 exposes it nicely in the thematic tutorials.