Clean up and fixes for migrate-to-built-in-kotlin guides

[parlough]

• Fixes the current build failure on main due to an incorrect link fragment.
• Add notes to the other guide at the top of each page.
• Removes unnecessary header nesting on both pages.
• Update link targets to follow our site standards.
• Move link definitions to the section they are used in.
• Slightly clean up some copy and wording.
• Uses contractions where our style guide suggests.
• Update some headings to be more consistent.
• Moves the issue template body into a Markdown code block and inline the links for one click copying.
• Adjust semantic line breaks.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 011ff13):

https://flutter-docs-prod--docs-pr13563-misc-migrate-to-built-bwjhigx7.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 011ff13):

https://flutter-dev-230821--www-pr13563-misc-migrate-to-built-soksio0t.web.app

[sfshaza2]

Wow! Thanks for cleaning this up. lgtm