updates in llms.txt

[AleksandrSherman]

• Removed root files that were fully overridden by every dialect

• Moved 4 shared pages (overview, data-querying, relations, relations-schema-declaration) into the singlestore dialect and deleted them from root _meta.json

• Added jit-mappers section to singlestore _meta.json for parity with other dialects

• Removed unused root files

• Updated logic in llms-full.txt

Strip the /pg prefix

• Updated logic in llms.txt

1. Strip /pg from generated links
2. Restructure headings: ## <Dialect display name> + ### <section> + links, replacing the old <dialect>/<section> format
3. Fix the Guides section, which previously emitted no individual guides