Add NetworkAttachmentConfig for service create/update

[hannseman]

The current documentation makes it appear like the only way to attach networks to a service is to provide them as a list of strings of ids/names.

Attaching networks also support setting aliases and driver options. I've added the new type docker.types.NetworkAttachmentConfig which is used to document these features.

I also found that a documentation link to ConfigReference did not work. It should be referenced as docker.types.ConfigReference, this is now fixed.

docker.types.ConfigReference was also referenced by its absolute path. I changed this to make the documentation a bit easier to read.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "network-attachment-config" git@github.com:hannseman/docker-py.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842357812144
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[hannseman]

Let me know if there's anything I can help with regarding this PR. Thanks!

I've also opened a PR in moby to fix the documentation there: moby/moby#39495

[shin-]

Sorry it took so long! LGTM - thank you!

[hannseman]

@shin- No problem! Thanks for the review.