Add docs for MFA feature

[Roma36]

Description of the change

This PR adds documentation for MFA feature.

Checklist

Please make sure that the proposed change checks all the boxes below before requesting a review:

• I have reviewed the guidelines for contributing to this repository.
• The preview looks fine.
• The tests pass.
• The commit history is clean and meaningful.
• The pull request is opened against the main branch.
• The pull request is no longer marked as a draft.
• You agree to license your contribution under the MIT license to Spacelift (not required for Spacelift employees).
• You have updated the navigation files correctly:
  • No new pages have been added, or;
  • Only nav.yaml has been updated because the changes only apply to SaaS, or;
  • Only nav.self-hosted.yaml has been updated because the changes only apply to Self-Hosted, or;
  • Both nav.yaml and nav.self-hosted.yaml have been updated.

If the proposed change is ready to be merged, please request a review from @spacelift-io/solutions-engineering. Someone will review and merge the pull request.

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Thank you for your contribution! 🙇

[render]

Your Render PR Server URL is https://spacelift-user-documentation-pr-396.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-clk96tl8td7s73e8hjj0.

[jmfontaine]

Except for the misplaced screenshots, it looks good to me.

[Roma36]

@jmfontaine I have addressed your comments and rebased this branch , but for some reason, one external link is failing check:
https://git.alpinelinux.org/aports/tree/testing/spacectl/APKBUILD returns 502

this has been fixed

[jmfontaine]

Looks good to me.