Remove registry groups from documentation

[danbarr]

Registry groups are being deprecated and are no longer surfaced by the CLI or UI when using the upstream registry format.

The custom-registry tutorial is being cleaned up in #814. Several other pages still document the feature and need to follow.

Pages to update

docs/toolhive/guides-cli/registry.mdx (~lines 238-317)

• Remove the entire ## Organize servers with registry groups section, including the registry JSON example with data.groups and the "Group structure" / "Full example" subsections
• The ### Use registry servers with groups subsection covers runtime groups (thv group create) which stay, but the section is currently nested under the registry-groups H2. Lift it to its own H2 (or fold it into a different section) so it isn't orphaned

docs/toolhive/guides-ui/registry.mdx (~lines 57-98)

• Remove the entire ## Registry groups H2, including the "Browse and view groups" and "Install a group" subsections

docs/toolhive/reference/registry-schema-upstream.mdx

• Drop the groups mentions from the front-matter description (line 5), the intro paragraph (line 17), and the data object description (lines 21-25). The full-example block already has no `groups` array

Out of scope

• blog/toolhive-updates/2025-12-08-updates.mdx - historical release note, leave as-is
• docs/toolhive/integrations/aws-sts.mdx - the `groups` mention there is a JWT claim, unrelated
• Runtime groups (`thv group create`, `--group` flag, group-management.mdx) are a different feature and stay

Acceptance

• No references to `data.groups` or registry-format groups in `docs/`
• Nav and links still resolve (no orphan anchors like `#registry-groups`)
• Build passes (`npm run build`)