Add a section for Tower

[evanfloden]

Include:

• how to login (concept of auth),
• how to add a pipeline (nextflow_schema.json)
• concepts of credentials
• concepts of compute environment
• concept of orgs and workspaces

[chriswyatt1]

This section is here: https://github.com/seqeralabs/nf-training-public/blob/master/asciidocs/tower.adoc

It is a simplification of tower docs with 1 example.

@drpatelh suggests that we link to the tower docs (that are updated), instead of a duplication. In which case, we need to decide what content goes in the tower section of the training. @evanfloden : I could follow the training video exactly of this section as a template?

[ewels]

Closing, as I think we're happy with how it is now.