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

docs/source/conf.py

@@ -448,7 +448,7 @@ with open("available_notebooks.rst", "w") as f:
             output = check_output(nb_help).decode('utf8')
             f.write(indent(output.replace("DETECTOR", detector).replace("TYPE", caltype), " "*4))
             f.write("\n\n")
-
+'''
 # add test results
 test_artefact_dir = os.path.realpath("../../tests/legacy/artefacts")