Document dash substitution for category slugs in shortcodes

[dgershman]

Summary

• Documents that category and tag names with spaces should use dashes in shortcode parameters (e.g., "Cell Awareness" → cell-awareness)
• Adds a general "Category and Tag Slugs" section to the admin shortcode docs table of contents
• Adds a [mayo_announcement_form] example for categories with spaces in their names

Closes #321

Test plan

• Open Mayo → Shortcodes in WordPress admin
• Confirm the "Category and Tag Slugs" note appears in the table of contents card
• Scroll to the [mayo_announcement_form] section and confirm the updated parameter descriptions and "Category with Spaces in Name" example

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