* Make the reader and validator understand links

Add the new category so that it can be read

* Make code generation create necessary link code

Effectively a header per link with a typedef and a common .cc file with
registration code for all links

* Make sure that SIO backend also works

* Separate link registration macros

* Remove special casing from roundtrip tests again

* Make sure to make link collections available

* Make sure that all user facing types exist

Otherwise things start to break in cases where users explicitly use
these types at the moment.

* Add documentation for new capabilities and links

* Update available templates documentation

* Only generate SIOBlocks if necessary

* Use walrus operator to improve readability

---------

Co-authored-by: Mateusz Jakub Fila <37295697+m-fila@users.noreply.github.com>
Co-authored-by: Andre Sailer <andre.philippe.sailer@cern.ch>