Tidy dockerfiles repo and packages

[sf1919]

Now that we have moved to conda packaging and are in the process of moving our Jenkins agents to new, slimmed down docker images it would be good to clean up the dockerfiles repo.

Major changes

• [ ] rename repo to jenkins-agents
• [ ] Create top level directory for Linux and move relevant folders
• [ ] Create top level directory for Windows and create relevant sub-folders
• [ ] Create top level directory for OSX

Remove directories that are no longer needed

• [ ] mantid/
• [ ] development/bin/

Delete any files not associated with new slim docker images from following directories

• [ ] development/docker/
• [ ] jenkins-node/mantid-builder/docker-images/
• [ ] development/docker/configure/

Assess function of following directories and their contents

• [ ] scripts/
• [ ] development/bin/
• [ ] jenkins-node/deployment/
• [ ] jenkins-node/mantid-builder/bin/

Update documentation

• [ ] Expand on top level readme.md to better describe the repo and it's contents
• [ ] Create readme docs for each OS folder and move exisiting information as appropriate

Tidy dockerfiles in packages

• [ ] Remove all but current and previous 2 docker images from the package repo - https://github.com/orgs/mantidproject/packages

[sf1919]

Top level directory for Windows will be created via #63