Add extensions contracts to docgen

[chmanie]

This PR adds the extension contracts to the docgen script. As they are not pure Interface contracts, some adjustments to the docgen script were necessary (inspection of nodes to be actual method nodes and checking of their visibility).

Note: the docgen script will now fail (exit with 1) if the docs couldn't be properly parsed in one or more instances (before they were just logged as warnings). Because we try to build the docs on CI as well now, this will make the CI tests fail if the docs can't be built.

Furthermore I fixed the issues raised in #1056 and #1057.

Closes #1056.
Closes #1057.
Closes #1036.

[CLAassistant]

All committers have signed the CLA.

[chmanie]

I am not sure why now some tests are failing. Can someone with more experience look into this please?

[area]

Okay, so the changes I've done here:

• Improve the versioning script. It now considers only (functional) bytecode changes, which is what actually matters. That should mean that any doc changes going forward don't require version bumps. And indeed, I've reverted the version bumps that were in this PR as a result.

• The downside of my changes is that checks out the old version and builds it; this makes the script inappropriate to be run in a hook, so I've removed it from there.

• Made a test reliant on version number of an extension more resilient.

• Reverted the one functional contract change (changing the visibility of createClaimDelayAction). I agree that should probably be done, but not here, in a doc-change PR.

Broadly speaking I'm now fine with the state of this, but I'd like @kronosapiens's input on the versioning script and if he thinks it could be improved / scenarios that it misses.

[kronosapiens]

Is there any handling if/when we move into a world where we're dealing only with the npm versions of the repo?