Update docs and create a proper user guide (fatiando#305)

Update Sphinx docs using sphinx-panels.
Add a proper User Guide that will ultimately replace the gallery examples.
Each page of the new User Guide is a .rst file that uses jupyter-sphinx to run example code blocks.
Added pages for: Coordinate systems, Forward Modelling, Gravity corrections and Equivalent Sources.
Added a new doc/versions.rst file with links to previous documentations.

.gitignore

@@ -11,7 +11,6 @@ dist/
 doc/_build
 doc/api/generated
 doc/gallery
-doc/tutorials
 doc/sample_data
 .ipynb_checkpoints
 *.egg-info

README.rst

@@ -40,12 +40,13 @@ or send us a message on our
 About
 -----
 
-*Harmonica* is a Python library for processing and modeling gravity and magnetic data.
-It includes common processing steps, like calculation of Bouguer and terrain
-corrections, reduction to the pole, upward continuation, equivalent sources, and more.
-There are forward modeling functions for basic geometric shapes, like spheres, prisms,
-polygonal prisms, and tesseroids. The inversion methods are implemented as classes with
-an interface inspired by scikit-learn (like `Verde <https://www.fatiando.org/verde>`__).
+*Harmonica* is a Python library for processing and modeling gravity and
+magnetic data. It includes common processing steps, like calculation of Bouguer
+and terrain corrections, reduction to the pole, upward continuation, equivalent
+sources, and more. There are forward modeling functions for basic geometric
+shapes, like point sources, prisms and tesseroids. The inversion methods are
+implemented as classes with an interface inspired by scikit-learn (like `Verde
+<https://www.fatiando.org/verde>`__).
 
 
 Project goals

doc/_static/custom.css

@@ -4,3 +4,23 @@ span.project-version {
   font-weight: 600;
   margin-left: 5px;
 }
+
+/* Try to make the jupyter-sphinx style match sphinx-gallery for consistency */
+div.jupyter_container {
+  border: none;
+  box-shadow: none;
+}
+
+div.jupyter_container .cell_input {
+  margin-bottom: 1.15rem;
+  border-radius: 0.4em;
+  box-shadow: 1px 1px 1px #d8d8d8;
+}
+
+div.jupyter_container .cell_output {
+  margin-bottom: 1.15rem;
+}
+
+.jupyter_container div.code_cell pre {
+  padding: 10px;
+}