EPIC: Migrate to using developer portal by default

[tolzhabayev]

Why? Since we now have a proper URL you can use to link to scenes documentation - we should migrate away from using github pages (github.io) and use the new URL instead.

DEV: https://grafana-dev.com/developers/scenes/ PROD: https://grafana.com/developers/scenes/

What to consider and how do we do this?

• Start by adjusting the inbound links to https://grafana.com/developers/scenes/
• Stop publishing to github.io
• Redirect from github.io (if you want to)
• Adjust the build scripts and remove the parent / landing page
• Remove github.io publishing workflows
• At some point in the future disable github pages

### Tasks
- [ ] Adjust inbound links to https://grafana.com/developers/scenes/
- [ ] Adjust the build scripts and remove the parent / landing page (similar to https://github.com/grafana/plugin-tools/pull/359)
- [ ] https://github.com/grafana/scenes/issues/296
- [ ] Add "back to dev portal" link - similar to https://github.com/grafana/plugin-tools/pull/377

[tolzhabayev]

@dprokop I can help out with the docusaurus config if you want - just let me know.