MRG: update with misc animal genomes (#3422)

Adds common hosts and also hg38.

Tackles #2717

<img width="667" alt="Screenshot 2024-12-07 at 9 17 40 AM"
src="https://github.com/user-attachments/assets/bfeff595-1759-4569-8adb-1e950f75a03e">

## Rendered preview:

doc/databases.md

@@ -30,6 +30,25 @@ The databases do not need to be unpacked or prepared in any way after download.
 
 You can verify that they've been successfully downloaded (and view database properties such as `ksize` and `scaled`) with `sourmash sig summarize <output>`.
 
+## Sketches for human and animal genomes
+
+These sketches are of the latest releases of a number of animal
+genomes. Among other uses, they can be used to detect host
+contamination in microbial metagenomes.
+
+Each file includes sketches at k=21, k=31, and k=51, at a scaled of
+1000, and is about 110 MB.
+
+* Human (hg38) - [hg38.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/hg38.sig.zip)
+* Cow (bosTau9) - [bosTau9.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/bosTau9.sig.zip)
+* Dog (canFam6) - [canFam6.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/canFam6.sig.zip)
+* Horse (equCab3) - [equCab3.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/equCab3.sig.zip)
+* Cat (felCat9) - [felCat9.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/felCat9.sig.zip)
+* Chicken (galGAl6) - [galGal6.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/galGal6.sig.zip)
+* Mouse (mm39) - [mm39.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/mm39.sig.zip)
+* Goat (oviAri4) - [oviAri4.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/oviAri4.sig.zip)
+* Pig (susCr11) - [susScr11.sig.zip](https://farm.cse.ucdavis.edu/~ctbrown/sourmash-db/host/susScr11.sig.zip)
+
 ## GTDB R08-RS214 - DNA databases
 
 [GTDB R08-RS214](https://forum.gtdb.ecogenomic.org/t/announcing-gtdb-r08-rs214/456) consists of 402,709 genomes organized into 85,205 species clusters.