Proximal subsampling tutorial

[jameshadfield]

Part of nextstrain/augur#1985

This was originally part of a nextstrain.org blogpost nextstrain/nextstrain.org#1362 but we decided to split it out into a page of its own.

This PR should be merged back-to-back with nextstrain/nextstrain.org#1362. There is a link to https://nextstrain.org/blog/2026-08-19-proximal-subsampling, this needs to be checked to ensure it's the correct datestamp.

[joverlee521]

Thanks for splitting this out! Flagged a couple small formatting issues, but overall looks good to me.

[joverlee521]

In the overall flow of docs, my initial thought is this fits better within or after the Filtering and Subsampling guide. However, I get that this is a common pattern that we'd like to widely surface to public health users.

Maybe a good in between is to link to the "Filtering and Subsampling" page in a "Next Steps" section so that users know where to go if they want more detail guide on subsampling. In Complex subsampling strategies, we can also link back to this tutorial for a walk-through of proximity subsampling.

[jameshadfield]

Yeah, I see that. I've moved it to a new page following filtering and subsampling, and added links between the two. I don't love how the how-to guides are hidden by default in the sidebar, but it is the better place for this. I've reworded things a little to call it a "how-to guide" not a "tutorial", but some usage of tutorial remains.