Upgrade to pymdownx's new Blocks API

[felker]

Mentioned in #601 (comment)

pymdownx introduced a new Blocks API as experimental in v9.10 (2023-03-05) and moved it out of beta in 10.12 (2024-10-28)
, while marking some of the older API's extensions (Details, Tabbed) as "legacy". There are a handful of advantages to the new API, but the chief benefits are that 1) it is more extendable, and 2) whitespace indentation is no longer necessary when writing them, especially helpful for nested blocks.

The first step would be to modify mkdocs.yml, i.e.

user-guides/mkdocs.yml

Lines 322 to 351 in 03b4d6c

	markdown_extensions:
	- footnotes
	- admonition
	- codehilite
	- tables
	- attr_list
	- md_in_html
	- def_list
	- pymdownx.critic
	- pymdownx.inlinehilite
	- pymdownx.keys
	- pymdownx.caret
	- pymdownx.mark
	- pymdownx.tilde
	- pymdownx.snippets:
	base_path: ["."] # for root of repo, default since that is where mkdocs.yml is
	# base_path: ["docs"]
	dedent_subsections: True # experimental; new in 9.10 (2023-03-05)
	check_paths: True
	url_download: True
	- pymdownx.superfences
	- pymdownx.details
	- pymdownx.inlinehilite
	- pymdownx.highlight:
	use_pygments: true
	anchor_linenums: true
	- pymdownx.tabbed:
	alternate_style: true
	- pymdownx.tasklist:
	custom_checkbox: true

would be changed to

 markdown_extensions: 
   - footnotes 
   - codehilite 
   - tables 
   - attr_list 
   - md_in_html 
   - def_list 
   - pymdownx.critic 
   - pymdownx.inlinehilite 
   - pymdownx.keys 
   - pymdownx.caret 
   - pymdownx.mark 
   - pymdownx.tilde 
   - pymdownx.snippets: 
       base_path: ["."]  # for root of repo, default since that is where mkdocs.yml is 
       # base_path: ["docs"] 
       dedent_subsections: True  # experimental; new in 9.10 (2023-03-05) 
       check_paths: True 
       url_download: True 
   - pymdownx.superfences 
   - pymdownx.inlinehilite 
   - pymdownx.highlight: 
       use_pygments: true 
       anchor_linenums: true 
   - pymdownx.tasklist: 
       custom_checkbox: true 
   - pymdownx.blocks
   #- pymdownx.blocks.admonition
   #- pymdownx.blocks.caption
   #- pymdownx.blocks.definition
   #- pymdownx.blocks.details
   #- pymdownx.blocks.html
   - pymdownx.blocks.tab
       alternate_style: true 

• Change mkdown.yml extensions as above
• Perform bulk replacement of !!! Admonitions and ??? Details fence syntax with ///
• Double check that this new pymdownx API remains fully compatible with mkdocs-material
• Try new / create ALCF-customized extension functionality enabled under the new API