Add documentation for 'PreviewHandler'

[sdirix]

• Document 'PreviewHandler'
• Add extension guide to README.md

Signed-off-by: Stefan Dirix sdirix@eclipsesource.com
Contributed on behalf of STMicroelectronics

Closes #8624

What it does

Add API documentation and extension guide to the preview package.

How to test

No need to specifically test as only documentation was changed

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

@sdirix thank you for the contribution, and your efforts in improving the documentation.
I have a few comments, namely about the wording, and consistency between doc updates.

Please also take the time to fix the build errors caused by extra trailing spaces.

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With all these welcomed documentation updates, I'm wondering if we should enforce a certain doc style with the use of eslint and adopting a certain style guide. It may be worthwhile to update the repo to enforce the style, perhaps as warnings initially but it did get me thinking 👍.

[sdirix]

With all these welcomed documentation updates, I'm wondering if we should enforce a certain doc style with the use of eslint and adopting a certain style guide. It may be worthwhile to update the repo to enforce the style, perhaps as warnings initially but it did get me thinking

I agree, a style guide would be very beneficial. Especially when it's encoded in ESLint.

[vince-fugnitto]

Thank you for contributing the documentation updates! 👍