Add OpenGraph social preview

[AmintorDusko]

Context: When posting links to the docs on social media, there is no social media preview, as there was no OpenGraph metadata generated by Sphinx.

Description of the Change:

• Adds matplotlib and sphinxext-opengraph as doc requirements

• configures conf.py to set the OpenGraph information, in particular,

  • the default social media preview image
  • uses pennylane_lightning.png as the preview image

Benefits: Better social media coverage.

Preview

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (502fecb) 98.96% compared to head (fb1b2ab) 99.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #574      +/-   ##
==========================================
+ Coverage   98.96%   99.09%   +0.12%     
==========================================
  Files         167      201      +34     
  Lines       24132    29203    +5071     
==========================================
+ Hits        23883    28938    +5055     
- Misses        249      265      +16     

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[josh146]

Thanks @AmintorDusko!

[AmintorDusko]

[sc-50785]

[multiphaseCFD]

Nice work! Thanks @AmintorDusko !

[vincentmr]

Looks good to me, thanks @AmintorDusko !