[Migrated] document the cocli template format

[deeglaze]

From veraison/corim#24

There has to be clear documentation around the template format used by:

CoRIM
Meta
CoMID
(Co)SWID

[ravjot07]

@deeglaze can i get some heads up like what does this documentation expects to be covered in..

[deeglaze]

I'll note that I've asked a similar question to the project heads in veraison/docs#23 but I think for this issue, a docs/templates.md file would be sufficient.