URL for documentation is inconsistent

[rbreslow]

• If you click Go To Docs, you get https://geotrellis.readthedocs.io/en/latest/.
• If you click Core Concepts, you get https://docs.geotrellis.io/en/latest/guide/core-concepts.html.

[alexelash]

Will note for future, but closing for now as the differing base slug doesn't affect the user's experience.