MRG: revise documentation structure; add internals page. (#2184) #3906

Job	Run time
Check	42s
coverage	4m 15s
Lints (beta)	45s
Lints (stable)	54s
Check if cbindgen runs cleanly for generating the C headers	2m 0s
minimum_rust_version	6m 4s
Publish (dry-run)	48s
test (beta)	2m 38s
test (stable)	2m 57s
test (windows)	5m 7s
test (macos)	3m 23s
test_all_features	15m 55s
Run tests under wasm32-wasi	2m 14s
Check if wasm-pack builds a valid package for the sourmash crate	2m 15s
	49m 57s

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