add redirect also for pyslim

[petrelharp]

@bhaller asks:

I notice that on https://tskit.dev/software/ the “Documentation” links are a bit inconsistent.  In particular, tskit’s docs are https://tskit.dev/tskit/docs/, msprime’s docs are https://tskit.dev/msprime/docs/, tsinfer’s docs are https://tskit.dev/tsinfer/docs/ – but pyslim’s docs are https://tskit.dev/pyslim/docs/stable, and indeed, https://tskit.dev/pyslim/docs/ does not even seem to work (!), but gives a -404.

This ought to fix that?

[benjeffery]

Thanks @petrelharp - the jsonschema issue you also fixed has been happening across all our doc builds.