add gather implementation benchmarks to sourmash docs

[ctb]

hackmd: link

gather benchmarking - sourmash v4.8.10 / branchwater plugin v0.9.5

Source repo: sourmash-bio/2024-benchmark-gather

sample SRR1976948

This sample contains 177 genomes.

Benchmarking results with 64 threads (note pygather uses 1).

prefix	s	max_rss
fastmultigather_rocksdb	102.103	515.24
fastgather	152.312	13071.1
fastmultigather	441.748	13029.6
pygather	2768.48	13755.2

Notes:

• Memory consumption is the same for all non-rocksdb implementations.
• fastgather is much faster than the others!

These trends held across all four samples.

rocksdb indexing of GTDB rs214

Indexing GTDB rs214 (400k sequences) took 4h 47m (17255 s) and 14 GB. The rocksdb index is 7 GB.