docs: add stackblitz playground for theming

[Coly010]

Description

Add stackblitz for theming and manual dark mode docs for Angular and React

Link to stackblitz
Angular - https://stackblitz.com/edit/ionic-ng-theming
React - https://stackblitz.com/edit/ionic-react-theming

Screenshots

[averyjohnston]

Thank you for the PR! We greatly appreciate the work you've put in. In the time since this was posted, we've updated the docs with playgrounds/live demos that include code for all major frameworks Ionic supports. (You can see examples on the component docs, such as here.) Rather than link to additional sandboxes, we should replace the ones on this page with equivalent playgrounds. This will also include example code for Vue.

I've created #2998 to track this work effort, and raised it internally with the team. I've also made sure to add a note about your PR so that we can give you co-author credit when adding the new demos.

I'm going to close this out in lieu of the above changes, but let me know if you have any questions, and thanks again.