Update RTD documentation. (docstrings, changelog, documentation refactors)

Description

1. Update Installation instructions. (Based on README.md)
2. Remove AGIPD64k from available detectors to calibrate.
3. Add changelog.rst
4. Expose cal_tools and xfel_calibrate functions' docstrings
5. Refactor the documentations.

How Has This Been Tested?

Relevant Documents (optional)

Types of changes

• Docs (changes to the documentation)
• Refactor (refactoring code with no functionality changes)

Checklist:

• I have updated the documentation accordingly.

Reviewers

@calibration