[docs] Rewrite SphinxQuickstartTemplate.md in MyST Markdown (#198896)
Converts SphinxQuickstartTemplate.md from reStructuredText syntax to
MyST Markdown. Updates the overview to mention that MyST is now
preferred for new LLVM documentation, modernizes the authoring
guidelines, and replaces all RST markup constructs with their Markdown
equivalents.
The renamd landed as PR #198899.
See RFC:
https://discourse.llvm.org/t/rfc-make-myst-markdown-the-llvm-docs-format-rip-rest/90840
An LLM was used to assist with the rewrite.
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Co-authored-by: Jonas Devlieghere <jonas@devlieghere.com>