[core] fix(SectionCard): rename component, fix Sass variable names

[adidahiya]

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

• Rename SectionPanel to SectionCard
  • ⚠️ this is not a breaking change since we haven't released [core] feat: Section component #6245 yet
• Rename Sass variables in _section.scss to match naming used in other parts of the Sass codebase (_tooltip.scss, button/_common.scss)
• feat(Section): add support for elevation prop which accepts a subset of the possible Elevation values (ZERO and ONE)

Reviewers should focus on:

Naming for the SectionCard component. It's a bit tricky to name container elements like this, but I feel like this is probably the best option for now. It's not:

• a panel, since those can be more complex components with different kinds of content inside (we expect Section contents to be a little simpler). also, this creates some ambiguity with PanelStack.
• "content" (as was the initial name in [core] feat: Section component #6245), since "content" is provided by Blueprint users, not Blueprint itself. the component is a container for content

references:

• https://ux.stackexchange.com/questions/78798/whats-the-difference-between-cards-panels-and-tiles

Screenshot

[qcz]

The file itself was not renamed.

[adidahiya]

good catch! thankfully that does not affect the public API, so I can fix that without a breaking change.