[WIP] revise documentation structure; add internals page.

[ctb]

This PR rearranges the docs to according to the https://diataxis.fr/ structure, per https://github.com/sourmash-bio/sourmash/issues/2054.

New pages:

• A heavily revised index page
• A guide to the internals of sourmash

---

A screenshot of the proposed new index page is below:

[codecov[bot]]

Codecov Report

Merging #2184 (a1ec24b) into latest (647bb8e) will increase coverage by 7.35%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #2184      +/-   ##
==========================================
+ Coverage   84.81%   92.17%   +7.35%     
==========================================
  Files         131      100      -31     
  Lines       15621    11345    -4276     
  Branches     2238     2238              
==========================================
- Hits        13249    10457    -2792     
+ Misses       2082      598    -1484     
  Partials      290      290              

Flag	Coverage Δ
python	92.17% <ø> (ø)
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/src/ffi/signature.rs
src/core/src/index/linear.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/index/mod.rs
src/core/src/sketch/minhash.rs
src/core/src/index/search.rs
src/core/src/ffi/mod.rs
src/core/src/ffi/cmd/compute.rs
src/core/tests/minhash.rs
src/core/src/cmd.rs
... and 21 more

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