Update reference documentation on HAL Explorer after upgrade to 1.1

[toedter]

I have released HAL Explorer 1.1.0 and updated the dependency. I also improved the HAL Explorer documentation a bit by adding links to git repo, reference doc and demo.

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes. => N/A
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only). => N/A

[odrotbohm]

Thanks for the submission, Kai! I've missed the PR before I created and fixed #2122. I still applied the rest of the PR for the documentation update.