DOC: Add texplanation for the tike.ptycho coordinate system

[carterbox]

Purpose

Provide ASCII art and text to explain how the tike coordinate system and edge padding work for ptychography.

Approach

Pre-Merge Checklists

Submitter

• [ ] Write a helpfully descriptive pull request title.
• [ ] Organize changes into logically grouped commits with descriptive commit messages.
• [ ] Document all new functions.
• [ ] Click 'details' on the readthedocs check to view the updated docs.
• [ ] Write tests for new functions or explain why they are not needed.
• [ ] Address any complaints from pep8speaks.

Reviewer

• [x] Actually read all of the code.
• [x] Run the new code yourself; the included tests should make this easy.
• [x] Write a summary of the changes as you understand them.
• [x] Thank the submitter.