Add badges to docs

What

• Last item of the new badges implementation - meta https://gitlab.com/gitlab-org/gitlab-ce/issues/42631
• Follows https://gitlab.com/gitlab-org/gitlab-ce/issues/40350
• Add product tier badges to docs:

Why

To facilitate the comprehension of which feature is available in which tier.

Architecture

• CE docs: add them only once, valid for the entire page
• Combined docs (different tiers for each section of the doc): apply them with Kramdown custom classes.

What we need

• Kramdown (https://gitlab.com/gitlab-com/gitlab-docs/issues/50)
• Code implementation
• BE/FE Review

cc/ @axil