Add paragraph to home page that outlines who Charmed HPC is useful for

[NucciTheBoss]

Another discussion that came out of #27. We're adding an admonition the notes what a cloud is in the context of Charmed HPC - something that provides compute, networking, storage, and so on - to dispel the misconception that Charmed HPC only works with public clouds like AWS and Azure.

@AshleyCliff mentioned that we should also dispel this misconception on the home page, but not be defensive about it. We should add a fourth paragraph to the home page which outlines who Charmed HPC is useful for:

You should consider using Charmed HPC if you want to take control of where your HPC infrastructure is deployed, or if you want manage your HPC clusters in an efficient, cost-effective way.

The above example needs a bit of wordsmithing, but we can use this fourth paragraph to further sell the benefits of using Charmed HPC such as not needing to budget for long procurement times and provision resources on-demand quickly.