Add dbt Mesh course link to the mesh docs and guides

[mirnawong1]

Contributions

• I have read the contribution docs, and understand what's expected of me.

Link to the page on docs.getdbt.com requiring updates

The training team has launched a new dbt Mesh course! It might be a good idea to promote this in our [dbt Mesh guide](https://learn.getdbt.com/courses/dbt-mesh), best practices doc, and cross project ref doc

What part(s) of the page would you like to see updated?

https://docs.getdbt.com/docs/collaborate/govern/project-dependencies#how-to-write-cross-project-ref
https://docs.getdbt.com/best-practices/how-we-mesh/mesh-4-implementation
https://docs.getdbt.com/best-practices/how-we-mesh/mesh-4-implementation

Additional information

relevant slack thread