Standardize per-machine hierarchy of sidebar sections/directory names/etc; limit max depth of sidebar hierarchy

[felker]

{Polaris, Sophia, Aurora, Crux} wont have a complete overlap of documentation pages, but where they do, the directories and navigation sidebar titles should be named identically. Polaris is probably our currently closest superset:

❯ pwd
./docs/polaris
❯ tree -d
.
├── applications-and-libraries
│   ├── applications
│   └── libraries
├── build-tools
├── compiling-and-linking
│   └── files
├── containers
├── data-science
│   ├── applications
│   └── frameworks
├── debugging-tools
├── hardware-overview
│   └── files
├── performance-tools
│   └── Images
├── programming-models
├── visualization
│   ├── images
│   └── scripts
└── workflows

• automate checking the symmetry in the names?

As seen in #554, renaming a directory after a significant amount of content has been added has the potential to be quite painful. It breaks all the relative links from .md files that are not children of that directory, plus other absolute links like extensions such as Snippets demands:

--8<-- "./docs/polaris/data-science/frameworks/pytorch.md:scalingsetup"

@FilippoSimini also pointed out that the current sidebar has way too many levels:

We could limit the max depth to 3 or so. Should probably revisit this after the significant style redesign in #545 is merged. Bolded "section headers" that don't increase the indentation or require clicking a dropdown arrow can help categorize things too.