Add documentation for Point oauth2

[fredrike]

Proposed change

https://github.com/home-assistant/core/pull/118243 implements use of HA's internal oauth2 authentication for Point this PR documents how to configure Point.

Type of change

• [ ] Spelling, grammar or other readability improvements (current branch).
• [ ] Adjusted missing or incorrect information in the current documentation (current branch).
• [ ] Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • [ ] I've opened up a PR to add logos and icons in Brands repository.
• [x] Added documentation for a new feature I'm adding to Home Assistant (next branch).
• [ ] Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase: https://github.com/home-assistant/core/pull/118243
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes #

Checklist

• [x] This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• [x] The documentation follows the Home Assistant documentation standards.

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[home-assistant[bot]]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks :+1:

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[fredrike]

Thanks for the suggestions @c0ffeeca7!