[Wiki]: Update Embedding-guide.md

[dixita0607]

Issue description

After adding "Copy to clipboard" functionality, this guide needs to be updated. When embedding the dartpad in an iframe, allow="clipboard-write" needs to be set for the copy to work.

Reference: Chromium's documentation on permissions in cross-origin iframes

Proposed changes to the wiki

The form of embedding previously documented here is no longer supported by DartPad.

You can still use DartPad's gist support to embed a DartPad as a normal iframe:

<iframe src="https://dartpad.dev/?id=5c0e154dd50af4a9ac856908061291bc?theme=light" allow="clipboard-write"></iframe>

Note: The allow="clipboard-write" permission is required for the copy to clipboard button to work in the embedded DartPad. See Chromium's documentation on permissions in cross-origin iframes for more information.

Follow #2702 for the future of DartPad embed support.