Add docs for how to create new symbolic variables metadata

[TorkelE]

I wrote a section in the Catalyst docs about metadata. I imagine some users might want to create there own. FOr this case, it would be neat to have a documentation of this in the Symbolics.jl docs, which I can link from the Catalyst.jl docs.

[jpfairbanks]

Is this the section you mentioned? https://docs.sciml.ai/ModelingToolkit/dev/basics/Variable_metadata/