Move some ECP-era workflows to community examples

[jlnav]

I propose for starters:

ytopt + heffte components/workflows

• ytopt proper hasn't been updated in 8 months (they just released 0.1.0), ytopt-libensemble (https://github.com/ytopt-team/ytopt-libensemble) is being sparsely maintained, mostly readme adjusts.
• they're operating off a fork for their purposes, among many other forks.
• All their supported machines are technically deprecated.

In general, I'd like to keep emphasizing that we don't need to build/test others' code to prove compatibility if it doesn't cover/test some specific feature of libEnsemble, in much the same way that numpy doesn't test/build libE to prove that numpy works (although maybe that would be appreciated 😄 ).

[jlnav]

Currently copied over:

• persistent_ax_multitask
• ytopt_asktell and heffte