docs(features): Add single instance page

[naman-crabnebula]

What kind of changes does this PR include?

• New or updated content

Description

• Closes https://github.com/tauri-apps/tauri-docs/issues/1489
• What does this PR change?
  • Adds the guide to use Tauri Single Instance Plugin

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[naman-crabnebula]

This PR is ready to be reviewed.

[naman-crabnebula]

Hi, up for review again! Thanks a lot for your valuable suggestions. I tried my best to accommodate most of the suggestions, and made the guide as beginner friendly as possible. I hope it is nice and clear!