[Docs] Add Privacy Policy link to the footer (#4435)

## Summary

* Adds a Privacy Policy link in the docs site footer
* Centers footer copyright content
* Fixes footer bottom padding and landing footer spacing across
breakpoints

## Test Plan

- [ ] Confirm the footer shows the Privacy Policy link and opens
`https://swmansion.com/privacy/policy/` in a new tab
- [ ] Check copyright text is centered on desktop and mobile
- [ ] Verify footer padding looks correct at common widths (desktop,
tablet, phone)
- [ ] On the landing page, confirm footer/background spacing still lines
up after the layout tweaks
<img width="1134" height="763" alt="image"
src="https://github.com/user-attachments/assets/adf47d83-3aa0-4130-9d48-88682723871a"
/>

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Co-authored-by: Artur Wojnar <arturwojnar@Arturs-MacBook-Pro.local>
Co-authored-by: Michał Bert <63123542+m-bert@users.noreply.github.com>