Dockerfile and Documentation Changes

[shihab-dls]

Documentation:

• Documentation is now in updated python-copier template.
• Documentation is split into three sections, representing the rootfs how to guides.
• File structure for the inclusion of overarching tutorials, references, how-to, and explanations sections (such as in PandABlocks.github.io repo) is maintained, in case new additions to these sections is made in the future. These files are excluded in conf.py (to allow for build without references to them); thus, this must be amended for new additions.

Dockerfile:

• Dockerfile now supports IRIS runners.
• Local build routine remains one-to-one with previous dev-container. Alpha release 3.1a3 is currently used on FPGA runners; thus, once Dockerfile merged, runners should run on pandablock-dev-container:latest.

[shihab-dls]

Empty sidebar removed by setting html_sidebar = { "***" = [] } for the included pages. Must be amended if more files are included...

[tomtrafford]

@coretl would you be able to have a look through this PR too? It's got prerequisite changes to these changes in the FPGA repo

[coretl]

A skim looks ok, I recomment asking @gilesknap about whether this is the best way to make the container into an actions runner. I would at least put it in a different stage so you don't get it all when you use the container as a dev container