MRG: revise documentation structure; add internals page. #3846

Job	Run time
Check	48s
test (beta)	2m 18s
coverage	4m 6s
Lints (beta)	46s
test (stable)	2m 11s
test_all_features	15m 24s
Lints (stable)	33s
test (windows)	5m 18s
Check if wasm-pack builds a valid package for the sourmash crate	1m 37s
test (macos)	3m 15s
Check if cbindgen runs cleanly for generating the C headers	1m 43s
minimum_rust_version	5m 37s
Publish (dry-run)	42s
Run tests under wasm32-wasi	1m 23s
	45m 41s

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