The docs-builder image build workflows have created some pain during
maintenance due to the way that the files have been managed, including:

* docs-builder image tag was calculated from all Documentation/ content.
  This means that _any_ change to Documentation files would cause the
  builder to need to be rebuilt.
* Documentation/Makefile contains the docs-builder image tag, meaning
  that any time we update the docs-builder tag, it updates the input
  used to calculate the docs-builder tag. As soon as you make one change
  to any docs file dependency, the docs-builder update GitHub workflows
  would always trigger the deployment to rebuild the docs-builder image,
  even if all changes to the docs-builder have been accounted for.
* Due to these discrepancies, there has been no way to detect whether
  the docs-builder needs updating, so the regular Documentation build
  did not depend on building a docs-builder image with the latest
  dependencies. This lead to cases where we would review and merge docs
  dependency updates without actually building the docs using the new
  dependencies.

Move all of the build infrastructure for creating the docs-builder
images under the images/ directory. By reducing the set of input files
for calculating the builder sha to only the minimal dependencies under
images/docs-builder directory, we can update the docs-builder only when
the dependencies update, rather than any time there is any change in the
documentation.

Co-authored-by: André Martins <andre@cilium.io>
Signed-off-by: Joe Stringer <joestringernz@gmail.com>

This new make target will check the docker tag for the docs-builder
image based on dependencies in the images/docs-builder/ directory, and
update the tag in all relevant locations if the tag would differ.

This target is only intended to create the diff so that there is a hard
dependency on the new builder image tag value, but it will not update
the $DOCS_BUILDER_IMG variable to the new fully specified image.

Add this new target to the 'builder-image' target so that any time the
builder image is updated, the files in the tree are also updated. This
should help to signal when the docs-builder references need to be
updated.

Signed-off-by: Joe Stringer <joe@cilium.io>