Update embedding sign-in-up form docs

[anku255]

Summary of change

There are 3 cases for embedding the sign-in-up form. This PR adds those 3 cases in the docs separately.

Checklist

• [ ] Algolia search needs to be updated? (If there is a new sub docs project, then yes)
• [ ] Sitemap needs to be updated? (If there is a new sub docs project, then yes)
• [ ] Checked for broken links? (Run cd v2 && MODE=production npx docusaurus build)
• [ ] Changes required to the demo apps corresponding to the docs?

Remaining TODOs for this PR

• [x] Update docs for other recipes

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