Document usage of CONDA_OVERRIDE_ARCHSPEC for microarch-level jobs?

### Where should the content be added?

https://github.com/conda-forge/conda-forge.github.io/blob/acf59ca87b09af1155b1f547a9133fb3bc37f8db/docs/maintainer/knowledge_base.md#microarchitecture-optimized-builds-microarch

### What should be added?

I have started experimenting with microarch optimized builds in https://github.com/conda-forge/mujoco-feedstock/pull/45, following the nice documentation added by @jaimergp in https://github.com/conda-forge/conda-forge.github.io/pull/2091 and https://github.com/conda-forge/conda-forge.github.io/pull/2094 .

If I follow the documentation, however, the osx builds on x86-64-v3 and both osx and linux on x86-64-v4 fail, as the CI machines do not have processor that respect that architecture, and `_x86_64-microarch-level` is being installed in the host environment during the build. See https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=892921&view=logs&jobId=d14e167c-fcd2-5062-49c0-597555c0b494&j=a94b0832-df3a-5403-f7c3-dd1e0f749138 for an example of failed build.

Perhaps we should also document a way to set `CONDA_OVERRIDE_ARCHSPEC` and skip tests in those cases?

### Additional information

Just to clarify, I am not expecting someone else to improve documentation, it is not not clear to me what is the best solution here.