Add first xplt example and floor plan

[eltos]

Description

Add a first example for xplt
following up on xsuite/xplt#5

See xsuite/xtrack#477 for related example code update

Checklist

Mandatory:

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

Optional:

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