docs: index Slack design entry

[2023Anita]

Summary

• Add Slack to the root collection index.
• Update the DESIGN.md count badge from 73 to 74.
• Add the missing design-md/slack/README.md redirect file.
• Fix the Slack frontmatter typo from Slacc to Slack.

Motivation

The repository already includes design-md/slack/DESIGN.md, but Slack was missing from the root README collection, the count badge still said 73, and the Slack directory did not have the standard per-design README redirect. This keeps the index metadata aligned with the shipped design files.

Verification

• Ran git diff --check.
• Checked that there are 74 design-md/*/DESIGN.md files.
• Checked every design slug has a root README getdesign.md link and a per-directory README.
• Checked Slack appears in the root README and the frontmatter uses Slack.

Risk

Docs-only/index-only change. No design tokens or runtime code are changed.