Add signup link to Artie integration docs

[danafallon]

What does this PR do?

Small, docs-only change for the Artie integration.

Motivation

We now support public signups and want to include the signup link in the docs (where we previously said to email us to start a trial).

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo
• If this PR includes a log pipeline, please add a description describing the remappers and processors.