Add first xplt example and floor plan

[eltos]

Description

Add a first example for xplt following up on https://github.com/xsuite/xplt/issues/5

See https://github.com/xsuite/xsuite/pull/487 for related docs update

Checklist

Mandatory:

• [ ] I have added tests to cover my changes
• [ ] All the tests are passing, including my new ones
• [x] I described my changes in this PR description

Optional:

• [x] The code I wrote follows good style practices (see PEP 8 and PEP 20).
• [x] I have updated the docs in relation to my changes, if applicable
• [ ] I have tested also GPU contexts