Create formal expresslangdoc

[TRThurman]

We seem to have consensus on having an expressdoc format defined. I checked the web and expresslangdoc is available.

[ronaldtse]

We have registered the expressdoc.org domain.

[ronaldtse]

And can start defining it now...

[TRThurman]

Where do we start?

[ronaldtse]

@TRThurman https://github.com/metanorma/annotated-express/issues/94