Add documentation for the concept of blocks, chunks, and sections

[yousefmoazzam]

The --max-cpu-slices flag requires some knowledge of these concepts but are not yet documented.

[yousefmoazzam]

279 adds docs which currently has a placeholder for when this information is referred to: https://github.com/DiamondLightSource/httomo/blob/fb85f54cb93abff1dee6a337f21e19c69a918fae/docs/source/howto/run_httomo.rst#--max-cpu-slices

[yousefmoazzam]

The "how to configure process lists" page contains a tentative header for "Platform sections" https://github.com/DiamondLightSource/httomo/blob/docs_update/docs/source/howto/process_lists/process_list_configure.rst#platform-sections.

Note that there are two options when writing the docs for sections:

• if documentation of sections ends up in that page, make sure to rename to "Section" (because the "platform" - CPU vs. GPU - is now not relevant to sections
• if documentation of sections goes somewhere else, then make sure to remove that unused header in the process lists page

[yousefmoazzam]

As mentioned in #282, the soon to be added/merged "Detailed concepts" section would be a good fit for this information https://github.com/DiamondLightSource/httomo/blob/26ed50c4c912021273a80dfcc6d35394c8cb23a8/docs/source/introduction/indepth/detailed_about.rst#detailed-concepts

[dkazanc]

thanks, moved around thing in this commit b8a96d1

[yousefmoazzam]

Completed in merge of #282