MRG: revise documentation structure; add internals page. #3904

Job	Run time
Check	47s
coverage	4m 28s
Lints (beta)	42s
test (beta)	2m 57s
Lints (stable)	57s
test (stable)	2m 45s
test_all_features	16m 9s
test (windows)	4m 53s
test (macos)	3m 21s
Run tests under wasm32-wasi	2m 3s
Check if wasm-pack builds a valid package for the sourmash crate	2m 15s
Check if cbindgen runs cleanly for generating the C headers	1m 41s
minimum_rust_version	5m 12s
Publish (dry-run)	47s
	48m 57s

Provide feedback