MRG: revise documentation structure; add internals page. #3843

Job	Run time
Check	35s
coverage	4m 10s
Publish (dry-run)	38s
Lints (beta)	41s
test (beta)	2m 22s
Lints (stable)	38s
test_all_features	22m 13s
test (stable)	2m 25s
Run tests under wasm32-wasi	2m 11s
test (windows)	5m 46s
Check if wasm-pack builds a valid package for the sourmash crate	1m 51s
test (macos)	2m 59s
Check if cbindgen runs cleanly for generating the C headers	2m 6s
minimum_rust_version	4m 37s
	53m 12s

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