Build docs with sphinx_copybutton and sphinxext.opengraph

[hugovk]

Changes proposed in this pull request:

• Add a couple of useful Sphinx plugins, as recommended by https://pradyunsg.me/furo/recommendations/

• sphinxext-opengraph: adds OpenGraph metadata (https://ogp.me), makes previews better on Facebook and Twitter (probably helps Google results too)

• sphinx-copybutton: adds a copy button to code snippets, to make it easy to copy them and try them out

• Plus use code formatting for some text on the installation page

Open graph

For example, These tags are inserted into the tutorial page:

<meta property="og:title" content="Tutorial" />
<meta property="og:type" content="website" />
<meta property="og:url" content="https://pillow.readthedocs.io/en/stable/handbook/tutorial.html" />
<meta property="og:description" content="Using the Image class: The most important class in the Python Imaging Library is the Image class, defined in the module with the same name. You can create instances of this class in several ways; e..." />

https://pillow--5498.org.readthedocs.build/en/5498/handbook/tutorial.html

Can compare what Facebook extracts from the PR:

https://developers.facebook.com/tools/debug/?q=https%3A%2F%2Fpillow--5498.org.readthedocs.build%2Fen%2F5498%2Fhandbook%2Ftutorial.html

with stable:

https://developers.facebook.com/tools/debug/?q=https%3A%2F%2Fpillow.readthedocs.io%2Fen%2Fstable%2Fhandbook%2Ftutorial.html

Copy button

The button at the top right copies the code block to the clipboard

https://pillow--5498.org.readthedocs.build/en/5498/handbook/tutorial.html#convert-files-to-jpeg

TODO

• Install plugins in .github/workflows/test.yml
• I'll update the PR to include the logo in og:image

[hugovk]

Added og:image:

https://developers.facebook.com/tools/debug/?q=https%3A%2F%2Fpillow--5498.org.readthedocs.build%2Fen%2F5498%2F

I hotlinked to https://github.com/python-pillow/pillow-logo/blob/master/pillow-logo-dark-text-1280x640.png using:

https://raw.githubusercontent.com/python-pillow/pillow-logo/master/pillow-logo-dark-text-1280x640.png

It's the right size, https://developers.facebook.com/docs/sharing/webmasters/images/ says:

• The minimum allowed image dimension is 200 x 200 pixels.
• The size of the image file must not exceed 8 MB.
• Use images that are at least 1200 x 630 pixels for the best display on high resolution devices. At the minimum, you should use images that are 600 x 315 pixels to display link page posts with larger images.
• We've also re-designed link page posts so that the aspect ratio for images is the same across desktop and mobile News Feed. Try to keep your images as close to 1.91:1 aspect ratio as possible to display the full image in News Feed without any cropping.