[MRG] add advanced database docs

[ctb]

Add information for advanced users.

Link to rendered doc: here

To fix:

• hidden toctree in index.md is not so hidden...

Fixes #1293.
Fixes #475.

[codecov]

Codecov Report

Merging #2025 (9493111) into latest (c1eea71) will increase coverage by 7.52%.
The diff coverage is n/a.

❗ Current head 9493111 differs from pull request most recent head 09d6d0a. Consider uploading reports for the commit 09d6d0a to get more accurate results

@@            Coverage Diff             @@
##           latest    #2025      +/-   ##
==========================================
+ Coverage   84.15%   91.67%   +7.52%     
==========================================
  Files         129       98      -31     
  Lines       15087    10807    -4280     
  Branches     2119     2119              
==========================================
- Hits        12696     9907    -2789     
+ Misses       2095      604    -1491     
  Partials      296      296              

Flag	Coverage Δ
python	91.67% <ø> (ø)
rust	?

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

Impacted Files	Coverage Δ
src/sourmash/minhash.py	93.98% <ø> (ø)
src/core/src/ffi/hyperloglog.rs
src/core/src/index/bigsi.rs
src/core/src/ffi/storage.rs
src/core/src/ffi/index/mod.rs
src/core/src/from.rs
src/core/src/storage.rs
src/core/src/errors.rs
src/core/src/ffi/mod.rs
src/core/src/index/mod.rs
... and 22 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c1eea71...09d6d0a. Read the comment docs.

[ctb]

Content is ready for review! @sourmash-bio/devs

[mr-eyes]

Looks so great! I enjoyed reading this documentation.