Added example for using ModelBuilder for pymc_experimental (pymc-devs#419)

* added model builder example notebook

* added examples/howto/Using_Model_Builder.ipynb

* after pre-commit

* fixed pre-commit

* passes all build tests

* Added installation for pymc-experimental

* updated markdown cell for installation guide

* 'removed .md files'

* passes build tests

* Update myst_nbs/howto/Model_Builder.myst.md

Co-authored-by: Thomas Wiecki <[email protected]>

* added before/after

* passes all tests

* fixed the updates

* fixed the updates

* updates

* removed build model from class method

* updated load

* final changes

* formating updates in first and last cell

* Update ModelBuilder docs.

* Update ModelBuilder docs.

* Update ModelBuilder docs.

* Incorporate feedback.

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Co-authored-by: Thomas Wiecki <[email protected]>

Author: 5hv5hvnk