Create separate file for adding-oauth-clients documentation

[nicholaspcr]

Summary

Closes #1136

Adds a separate page for adding an OAuth client. This basically introduces the TTSS considerations provided in the issue alongside an already existent documentation in https://www.thethingsindustries.com/docs/api/concepts/auth/oauth-access-tokens/

Screenshots

Changes

• Create page on integrations/adding-oauth-clients/
• Update content of api/concepts/auth/oauth-access-tokens/

Notes for Reviewers

This includes the image change where the Enabled checkbox in the grant section now displays the proper label value.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Run Locally: Verified that the docs build using make server, posted screenshots, verified external links. Test with HUGO_PARAMS_SEARCH_ENABLED=true if style changes will affect the search bar.
• New Features Marked: Documentation for new features is marked using the new-in-version shortcode, according to the guidelines in CONTRIBUTING.
• Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.

[KrishnaIyer]

This does not belong in integrations. That section is meant for users to get LoRaWAN traffic out of TTS or into TTS (downlinks)
I think we can put this in https://www.thethingsindustries.com/docs/the-things-stack/interact/