manifest: Add a new `description` project attribute

[carlescufi]

This PR adds a new description project attribute that contains an optional text description of the project.

[carlescufi]

@mbolivar-ampere before I start modifying/adding tests I'd like to discuss whether you would like the Project() constructor signature to look like. I currently have:

   def __init__(self, name: str, description: Optional[str], url: str,
                 revision: Optional[str] = None,
                 path: Optional[PathType] = None,
                 submodules: SubmodulesType = False,
                 clone_depth: Optional[int] = None,
                 west_commands: Optional[WestCommandsType] = None,
                 topdir: Optional[PathType] = None,
                 remote_name: Optional[str] = None,
                 groups: Optional[GroupsType] = None,
                 userdata: Optional[Any] = None):

but this will break existing apps. Should I instead place description as the last argument?

[carlescufi]

Interesting! I'm aware the west documentation is unfortunately still in a different repo, so could elaborate in the commit message or source comments about where this new field will be displayed and how? Truncated in the default west list maybe?

Right now I've only added to the repr() but ultimately this is really to be displayed in the manifest itself. Today it is hard to identify projects in large manifests with functionality, and by adding this field I am hoping that this will be easier.

[carlescufi]

Thanks for the review @mbolivar-ampere! Comments addressed.