docs: update migrate swift plugin docs with .gitignore

[tsinis]

Description of what this PR is changing or adding, and why:

This PR adds notes about new .gitignore entries for those who want to migrate their old packages to SPM.

Issues fixed by this PR (if any):

Without this step - the guide suggests "Commit your plugin's changes to your version control system" with those .build/ and .swiftpm/ directories newly created in previous steps.

PRs or commits this PR depends on (if any):

flutter/packages#6705

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[sfshaza2]

lgtm but please weigh in, @loic-sharma.

[sfshaza2]

/gcbrun

[sfshaza2]

/gcbrun

(Didn't mean to close it!).

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit c1d4931):

https://flutter-docs-prod--pr11497-patch-1-lcfxgrky.web.app

[sfshaza2]

Loic is on vacay, so landing.