EPIC: Migrate to using developer portal by default

[tolzhabayev]

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

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

What to consider and how do we do this?

• Start by adjusting the inbound links to https://grafana.com/developers/dataplane/
• 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

Nice to have

• Add "Tools & Examples" sections and page to the side navigation to not loose this links once we get rid of the dataplane specific landing page.

### Tasks
- [ ] Adjust inbound links to https://grafana.com/developers/dataplane/
- [ ] Adjust the build scripts and remove the parent / landing page (similar to https://github.com/grafana/plugin-tools/pull/359)
- [ ] https://github.com/grafana/dataplane/issues/33
- [ ] Add "back to dev portal" link - same as https://github.com/grafana/plugin-tools/pull/377

### Nice To Have
- [ ] Add "Tools & Examples" sections and page to the side navigation to not loose this links once we get rid of the dataplane specific landing page.

[tolzhabayev]

@yesoreyeram I can help out with this if you want - let me know.