docs: add mxbuild setup & mx tool skill (#43)

* docs: add mxbuild setup & mx tool skill for end users

Addresses common agent mistakes when guiding users through mxbuild
setup and mx tool usage, particularly incorrect mx create-project
syntax and missing mxcli docker check as the preferred validation
wrapper.

* fix: address review — remove overlapping mxbuild-setup skill

Remove mxbuild-setup.md (content already covered by docker-workflow.md)
and fold the non-overlapping caveat about blank projects lacking demo
users into docker-workflow.md.

Fixes factual error: mxbuild-setup.md incorrectly claimed mx
create-project does not accept --output-dir (verified it does in
mx v11.6.4).

Author: engalar

.claude/skills/mendix/docker-workflow.md

@@ -65,6 +65,8 @@ mkdir -p /path/to/my-app
 
 The `mx create-project` command creates an MPR v2 project with the standard Mendix module structure. You can then use the Docker workflow to build and run it.
 
+**Caveat:** Blank projects have no demo users — login will fail until you configure security via MDL or Studio Pro. See `manage-security.md` for setting up demo users.
+
 ## Step-by-Step Workflow
 
 If you prefer more control, use the individual commands: