docs: add VM instance compatibility matrix

[dsloanm]

Adds a compatibility matrix reference page for VM instance types that have been used for testing Charmed HPC on Azure.

At the moment, this is mostly beneficial for the slurmd charm, as this has been tested on a few different combinations of instances to confirm RDMA and GPU compatibility. The remaining charms have been exclusively run on Standard_D2as_v6 instances, Juju's default for Azure.

[AshleyCliff]

Looks good! Definitely some useful information to have available to users - we'll want to update as we test on other VMs. I've made just a few suggestions on phrasing and maybe a different title.

[AshleyCliff]

Something like 'VM compatibility' might be better as a title, since the current title doesn't hint at what is/is not compatible.

[dsloanm]

Good catch. I've changed the title to "Virtual machine compatibility" for clarity.

[AshleyCliff]

One suggestion about adding a note.

Once the release policy is confirmed, we'll likely want to either add a column noting which Charmed HPC release is being tested, or have a unique page/section for each release. Since the release policy should be implemented in near future, we should potentially leave this PR unmerged until those updates can be made. Thoughts?

[dsloanm]

Note has been added and I've rebased on current main once more.

Yes, I think that's a good idea to keep this un-merged for now. We can update when we have the release policy in place.