MRG: revise documentation structure; add internals page. #3889

Job	Run time
Check	48s
Check if cbindgen runs cleanly for generating the C headers	1m 44s
coverage	4m 12s
Lints (beta)	43s
minimum_rust_version	5m 14s
Lints (stable)	53s
Publish (dry-run)	49s
test_all_features	16m 2s
Run tests under wasm32-wasi	1m 58s
test (beta)	2m 44s
Check if wasm-pack builds a valid package for the sourmash crate	1m 50s
test (stable)	2m 24s
test (windows)	4m 28s
test (macos)	3m 57s
	47m 46s

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