feat: add mapping spec document 5006

[ronaldtse]

No description provided.

[TRThurman]

@ronaldtse @stuartgalt, I question whether we should take credit for semantics, as we don't provide sufficient information for a post-processor to easily evaluate whether the EXPRESS declarations are valid.
I intend to write a Jira issue requiring express links and changes to the 'source' element to reference the schema instead of the part.
With that change (ediition 2 of this document?) , I think it is safe to claim support for semantics.
thoughts?

[ronaldtse]

I started this document because I was trying to understand the mapping symbols and get a validator working.

The document is clearly in draft, so let's say it is "forward looking". In that sense, why don't we directly require the source being the schema (instead of part)? That makes lots of sense.

[TRThurman]

Thanks Ron I agree. Eventually we want to use express links instead of strings.