move custom accessors to advanced and add more complex, runnable examples (#168)

* add advanced/accessors section and remove accessors part 2 from xarray ecosystem
* add geoid offset ex
* add insar velocity accessor example

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

Author: 3 people

_toc.yml

@@ -60,6 +60,9 @@ parts:
         sections:
           - file: advanced/backends/1.Backend_without_Lazy_Loading.ipynb
           - file: advanced/backends/2.Backend_with_Lazy_Loading.ipynb
+      - file: advanced/accessors/accessors.md
+        sections:
+          - file: advanced/accessors/01_accessor_examples.ipynb
 
   - caption: Workshops
     chapters: