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

@@ -403,6 +403,9 @@ with open("available_notebooks.rst", "w") as f:
             """))
 
     for detector in sorted(notebooks.notebooks.keys()):
+        # Avoid having test notebooks in detector notebooks documentations. 
+        if "TEST" in detector.upper():
+            continue
         values = notebooks.notebooks[detector]
         f.write("{}\n".format(detector))
         f.write("{}\n".format("-"*len(detector)))
@@ -410,8 +413,6 @@ with open("available_notebooks.rst", "w") as f:
 
         for caltype in sorted(values.keys()):
             data = values[caltype]
-            if data.get("notebook", None) is None:
-                continue
             nbpath = os.path.abspath("{}/../../../{}".format(__file__, data["notebook"]))
             with open(nbpath, "r") as nf:
                 nb = nbformat.read(nf, as_version=4)