Update theming.mdx

[dylandbl]

Removed link in table of contents to non-existent Usage section.

What:
Updating theming.mdx to remove a dead link in the table of contents. The link was to a non-existent 'usage' section.

Why:
In its current state, the link does nothing. Just adds confusion.

How:
I removed one line from the theming readme file.

Checklist:

• Documentation
• Tests
• Code complete
• Changeset

[changeset-bot]

⚠️ No Changeset found

Latest commit: 45d956f

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Sandbox	Source
Emotion	Configuration