feat: add FormGroup component documentation to sistent components page

[Xynash]

Description
This PR closes #7663. Adds documentation for the FormGroup component to the Sistent components page.

Three MDX files created at src/collections/sistent/components/formgroup/:

• index.mdx — Overview and usage of FormGroup
• guidance.mdx — Best practices, layout variants, and accessibility guidelines
• code.mdx — Code examples with ThemeWrapper and SistentThemeProvider

Follows the same structure as existing components like Button and Card.

Notes for Reviewers

• Vertical and horizontal layout examples included
• Switch component usage documented
• Accessibility patterns with FormControl and FormLabel covered
• All CodeBlock snippets use SistentThemeProvider

Signed commits

• Yes, I signed my commits.

[github-actions]

Preview deployment for PR #7666 removed.

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[Xynash]

This PR is ready for review! All three tabs are rendering correctly on the preview .

[Bhumikagarggg]

@Xynash Thank you for your contribution! Let's discuss this during the website call tomorrow at 5:30 PM IST | 7 AM CST Add it as an agenda item to the meeting minutes, if you would 🙂

[Daniel-1600]

Hi @Xynash thanks for working on this issue, could you squash all your commits into one for easier review thanks!

[Xynash]

Hi @Xynash thanks for working on this issue, could you squash all your commits into one for easier review thanks!

Done! Squashed all commits into one .