Missing manual content regarding I/O

[eguiraud]

• mention writing to JSON other than XML
• Float16_t
• LinkDef syntax reference, which pragmas are available, what + and - do, ...
• I/O constructors & constructors used by ROOT I/O
• Create a new doxygen md, documenting genreflex, selection.xml and reference selection namespace doxygen page.

[olantwin]

Just ran into this issue while trying to refresh my memory of the exact syntax of LinkDefs (instead of just copy/pasting existing ones and experimentally adapting them).

Only the User Guide seems to contain documentation for all the syntax features I've seen in the wild, but it is not clear comparing with the Manual which parts still apply and which LinkDef statements are ignored. (With many example LinkDefs in out software stack still using statements which don't do anything anymore, if I'm not mistaken).

An additional point worth documenting might be whether anything special needs to be done to make classes accessible using PyROOT.

Sorry to dig out an old issue, but as this is an important part of ROOT, I just wanted to bring it back to memory.