Improve usage page design when no data is present (empty state)

[gtsiolis]

Problem to solve

Following up from #11554 (Cc @laushinka @easyCZ @jldec), the usage page empty state could benefit from the following changes:

1. Reused the existing pattern and design of the empty state used elsewhere in the product, see screenshots below.
2. Update the empty state copy for better clarity. What are sessions, how can you check with other teams, etc.

No sessions found.
Have you started any workspaces or checked your other teams?

Usage

Variables	SSH Keys

Workspaces	Projects

[svenefftinge]

I think this has been implemented.

[gtsiolis]

@svenefftinge Thanks for noticing! This issue was about improving the design of the empty state layout to match the pattern and style of other empty states across the product, not adding the empty state for previous months as described in #12803 and #12858.

But let's keep this closed and reopen a new one once the design of the empty state becomes a real issue. We'll probably also need to update this to improve the UX when users select custom ranges and no results are returned, etc.

In the meantime, I may open a PR for updating the design of this page.