Standardize whitespace and tabs in source for indentation-sensitive markup features

[felker]

A note from #640, specifically https://github.com/argonne-lcf/user-guides/pull/640/files#r1919104321

Admonitions can be very picky about indentation in order to render correctly:

The content of the block follows on the next line, indented by four spaces:

E.g. when using !!! note the block should begin on the next line with content with the first letter under the n (or later?)

A single tab character seems to still work, e.g. in:

user-guides/docs/aurora/running-jobs-aurora.md

Lines 408 to 410 in 695a3d0

	!!! warning
	If you want to `ssh` or `scp` to one of your assigned compute nodes you will need to make sure your `$HOME` directory and your `$HOME/.ssh` directory permissions are both set to `700`.

(and throughout that .md, it seems like a mix of whitespace+tabs, 6 whitespaces, etc. indentation for Admonitions all seem to render correctly in mkdocs-material...)

By default, GitHub renders tabs in Markdown rich previews as 8 spaces, so it can be hard to spot errors. And changing a GitHub user's default "number of spaces a tab is equal to when rendering code" from 8 to 4 via https://github.com/settings/appearance is not reflected everywhere, e.g. the above permalink snippet in this comment.

Tab characters would be better replaced by 4 whitespaces. Our AI proofreader seems to agree; it replaced the tab character below with 4 spaces in 8a62194

• replace all tab characters and prevent them from being introduced in future commits?
• Or, this is another reason to move to the new API (Upgrade to pymdownx's new Blocks API  #609), where Admonitions and Blocks generally are not whitespace-sensitive.

Related to #330 and the potential benefit from having automatically-enforced style rules.