Add a Deploy to Netlify button

[timwis]

I've had trouble getting the Deploy to Netlify button working. I think it's because it always uses the default branch (even though I can see things on google where people are using &branch=xyz, it doesn't seem to work). But more recently I'm getting complete failures when deploying, e.g.

11:14:48 AM: build-image version: a2d22d22e4555d1ef0a972ed14a0a4b366ad20c4 (focal)
11:14:48 AM: build-image tag: v4.16.3
11:14:48 AM: buildbot version: 211709fe0bb687fe4fe2ebbb0a0d7258aa6e265c
11:14:48 AM: Fetching cached dependencies
11:14:48 AM: Failed to fetch cache, continuing with build
11:14:48 AM: Starting to prepare the repo for build
11:14:48 AM: User git error while checking for ref refs/heads/netlify-deploy
11:14:48 AM: Failing build: Failed to prepare repo
11:14:48 AM: Failed during stage 'preparing repo': error checking for ref: : Host key verification failed.
fatal: Could not read from remote repository.Please make sure you have the correct access rightsand the repository exists.: exit status 128
11:14:48 AM: Finished processing build request in 453ms

I don't think this should hold up any releases though, as the documentation now covers deploying it manually.