MRG: revise documentation structure; add internals page. #3845

Job	Run time
Check	34s
test (beta)	2m 45s
test (stable)	2m 18s
test (windows)	5m 11s
test (macos)	3m 2s
Check if cbindgen runs cleanly for generating the C headers	1m 38s
coverage	4m 49s
Lints (beta)	42s
Lints (stable)	32s
minimum_rust_version	4m 43s
Publish (dry-run)	43s
test_all_features	19m 0s
Run tests under wasm32-wasi	1m 52s
Check if wasm-pack builds a valid package for the sourmash crate	1m 47s
	49m 36s

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