Release v0.15.6

Version 1.21 is archived and has unfixed CVEs.

[release-0.15]: bump dependencies

Update gh-pages has started failing with default permissions. It needs
write permissions to be able to push to the gh-pages branch.

…1906-to-release-0.15

[release-0.15] github: specify workflow permissions

The upstream repo (and the release downloads)
github.com/rundocs/jekyll-rtd-theme has been deleted. This broke our
docs generation as the remote theme configuration depended on
downloading the release artefact.

This patch changes the docs building to use a Ruby gem instead of the
remote theme setting. To complicate matters, the gem has an seemingly
incorrect (too strict) version dependency. To mitigate this, we now
install bundler-override plugin to ignore this particular dependency.

The netlify conf is a hack, but I wasn't able to figure out a way how to
install the bundler-override plugin without doing all ruby
initialization in the build command.

…1829-to-release-0.15

[release-0.15] docs: use jekyll-rtd-theme from a ruby gem

[release-0.15] Release v0.15.7