MRG: revise documentation structure; add internals page. #3869

Job	Run time
Check	30s
coverage	5m 57s
Lints (beta)	1m 1s
Publish (dry-run)	49s
Lints (stable)	1m 5s
test (beta)	2m 29s
test (stable)	2m 34s
test (windows)	6m 16s
Run tests under wasm32-wasi	1m 48s
test_all_features	15m 56s
test (macos)	3m 18s
Check if wasm-pack builds a valid package for the sourmash crate	1m 43s
Check if cbindgen runs cleanly for generating the C headers	2m 5s
minimum_rust_version	4m 42s
	50m 13s

Provide feedback