Remove nbsphinx extension

[maxrjones]

Description of proposed changes

This PR removes the nbsphinx extension because it is no longer needed after #268 and produces unnecessary warnings about duplicate files.

Fixes #920

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

More things should be done:

1. remove nbsphinx from environment.yml
   

   pygmt/environment.yml

   Line 26 in 0fd514e

   - nbsphinx

2. If you look at the old documentation (https://www.pygmt.org/0.0.1a0/tutorials/plot-data-points.html), you will see a "Download notebook" button at the top right. That was added by the template, and we no longer need it. So the following lines should be safe to remove. (These lines check if the suffix of the source file is .ipynb. If yes, then add the download button).

pygmt/doc/_templates/breadcrumbs.html

Lines 29 to 32 in 0fd514e

	{% if suffix == ".ipynb" %}
	<a href="https://github.com/{{ github_repo }}/raw/{{ github_version }}/{{ doc_path }}/{{ pagename }}{{ suffix }}"
	class="fa fa-download"> Download notebook</a>
	{% endif %}

and the top comments should also be updated accordingly:

pygmt/doc/_templates/breadcrumbs.html

Lines 1 to 2 in 0fd514e

	{# Extend the RTD template to include "Edit on GitHub" and option to download
	notebook generated pages from nbsphinx #}

[maxrjones]

More things should be done:

1. remove `nbsphinx` from `environment.yml`
   https://github.com/GenericMappingTools/pygmt/blob/0fd514e03c4e87515967140e8c3c47234380511a/environment.yml#L26

2. If you look at the old documentation (https://www.pygmt.org/0.0.1a0/tutorials/plot-data-points.html), you will see a "Download notebook" button at the top right. That was added by the template, and we no longer need it. So the following lines should be safe to remove. (These lines check if the suffix of the source file is `.ipynb`. If yes, then add the download button).

pygmt/doc/_templates/breadcrumbs.html

Lines 29 to 32 in 0fd514e

	{% if suffix == ".ipynb" %}
	<a href="https://github.com/{{ github_repo }}/raw/{{ github_version }}/{{ doc_path }}/{{ pagename }}{{ suffix }}"
	class="fa fa-download"> Download notebook</a>
	{% endif %}

and the top comments should also be updated accordingly:

pygmt/doc/_templates/breadcrumbs.html

Lines 1 to 2 in 0fd514e

	{# Extend the RTD template to include "Edit on GitHub" and option to download
	notebook generated pages from nbsphinx #}

Thanks, I made those changes.