Add central glossary, additional abbreviations, links, and minor editorial changes

[kafitzgerald]

This PR:

• Adds a central glossary that lives in the Appendix, but can be referenced from anywhere and removes page specific glossaries.
• Adds several additional abbreviations to the central configuration to fill that out a bit
• Adds several links to external resources
• Makes some minor editorial changes to "Why Python?"

Closes #569 and partially addresses #571 (central glossary)

More detailed notes / references:

• Another option for handling a longer glossary could be to do something like this: https://book.the-turing-way.org/afterword/glossary/
• MyST glossary, abbreviation, etc. docs here: https://mystmd.org/guide/glossaries-and-terms
• There are some cases where we'll need to make a call on if we'd like to see an abbreviation, a glossary term, fancy Wikipedia link, external link, or some combination thereof. Personally, I think the Wikipedia links are quite nice for background info / definitions that are a little less central to the Pythia Foundations content, but they do override the abbreviation feature IIRC and not everything has a Wikipedia entry (e.g. NCL). There's probably also a threshold for overuse with those. Anyway, very much open to suggestions / preferences.

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