MRG: add branchwater web to docs (#3018)

This PR updates the front page with a link to the branchwater SRA search
tool.

Link: [readthedocs build for this
PR](https://sourmash--3018.org.readthedocs.build/en/3018/)

Image:

<img width="690" alt="Screenshot 2024-02-20 at 7 35 22 AM"
src="https://github.com/sourmash-bio/sourmash/assets/51016/42ff3a8d-cd9e-442d-b7f6-0b4885500f3d">

doc/index.md

@@ -17,6 +17,10 @@ You might try sourmash if you want to -
 * taxonomically classify genomes or metagenomes against NCBI and/or GTDB;
 * search thousands of metagenomes with a query genome or sequence;
 
+<span style="color:red"><b>New!</b></span> The sourmash project also supports
+[querying all 1 million publicly available metagenomes in the Sequence Read
+Archive](https://branchwater.sourmash.bio/). Give it a try!
+
 Our **vision**: sourmash strives to support biologists in analyzing
 modern sequencing data at high resolution and with full context,
 including all public reference genomes and metagenomes.