fix: use ddev describe to check for a project

[stasadev]

The Issue

@rfay asked me to test this add-on, so here's my improvement for:

Unable to install the add-on, when you don't have a running project, because ddev exec true doesn't work in this case:

$ ddev stop
$ ddev add-on get UltraBob/ddev-drupal-code-quality
...
Executing post-install actions: 
Accept recommended settings? (y/N) y
ERROR: ddev exec could not resolve a project from /home/stas/code/ddev/d11/.ddev.
Run the installer from the target project (for add-on installs, this should be automatic).
Try: cd /home/stas/code/ddev/d11 && ddev exec true
👎  Install Drupal.org GitLab CI template configs and host shims (bash)

How This PR Solves The Issue

• Uses ddev describe instead of ddev exec true, which does the same thing in this situation.
• Updates the README with links to respective add-ons, and removes "If/when published to the add-on registry", since it's already published.

Manual Testing Instructions

ddev stop
ddev add-on get https://github.com/UltraBob/ddev-drupal-code-quality/tarball/refs/pull/22/head
ddev restart

Automated Testing Overview

Release/Deployment Notes