Fix gallery (#361)

* create truncated regression example

* delete truncated regression example from main branch

* create truncated regression example

* delete truncated regression example from main branch

* create truncated regression example

* delete truncated regression example from main branch

* fix incorrect statement about pm.NormalMixture

* update gallery to include all (relevant) subfolders of examples

* commit the *.myst.md files just magically created

* delete myst_nbs/pymc3_howto

* small fix to underlining of section headings

* rename pymc_howto/ to howto/

* add the howto/*.myst.md files that suddenly appeared

* rerun sampling_callback.ipynb (under v3) to fix cell order error

* remove duplicate section in gallery.rst

* global find and replace pymc3_howto -> howto

* remove another duplicate in gallery.rst 🙄

* move BART and Spline notebooks into Case Studies to fix gallery rendering error

It doesn't like it when there is only one notebook in each section

* this should do it!

* removing BART and Splines headings from gallery

* fix howto gallery

Co-authored-by: Benjamin T. Vincent <drbenvincent@users.noreply.github.com>
Co-authored-by: Oriol Abril-Pla <oriol.abril.pla@gmail.com>

Author: 3 people

examples/gallery.rst

@@ -48,16 +48,6 @@ Gaussian Processes
 
     gaussian_processes/*
 
-BART
-----
-
-.. nbgallery::
-    :name: BART
-    :glob:
-    :reversed:
-
-    BART/*
-
 Inference in ODE models
 -----------------------
 
@@ -126,14 +116,4 @@ How to
     :glob:
     :reversed:
 
-    howto/*
-
-Splines
--------
-
-.. nbgallery::
-    :name: splines
-    :glob:
-    :reversed:
-
-    splines/*
+    howto/*