Add opengraph tags

[manics]

Jupyter readthedocs sites are often referenced on the Jupyter Discourse. The lack of Open Graph tags means link previews aren't useful, e.g. in the compose mode

This uses sphinxext-opengraph as suggested by @choldgraf in pydata/pydata-sphinx-theme#664

[consideRatio]

@manics wieee nice!! I observe it to not yet function though based on the PR preview. I did this on discourse.jupyter.org:

It sais it fails to find image and description. But, I see the following though... Hmmm...

---

• I think we should set ogp_site_url to https://z2jh.jupyter.org to avoid use of the longer https://zero-to-jupyterhub.readthedocs.io/
• I think we should explicitly set ogp_image to https://z2jh.jupyter.org/en/latest/_static/logo.png
  
• I think we should set the ogp_use_first_image option to true

[choldgraf]

agree w/ @consideRatio - we need to add a bit of configuration to get the base url right etc, and then the link metadata / logo should show up properly

[manics]

I've added some tags for og:image. I didn't change the site URL because according to the docs RTD will set it automatically, and it may need to include the site prefix en/latest/ which may cause problems with multiple versions of the docs.

https://cards-dev.twitter.com/validator refuses to load the rendered RTD pages because bots are blocked by robots.txt, maybe Discourse does the same?

[manics]

The other option mentioned in pydata/pydata-sphinx-theme#664 (comment)

define a jupyterhub-sphinx-theme that pre-configures some things for all of our repositories. For example, we could automatically add a link to our Discourse forum, Jupyter home page, etc (and each repository could then additionally configure it on top of that).

If we can include

ogp_image = "_static/logo.png"
ogp_use_first_image = True

in jupyterhub-sphinx-theme and all JupyterHub RTD sites have _static/logo.png this would provide a drop-in zero config theme. We could also pin the version of pydata-sphinx-theme exactly but leave jupyterhub-sphinx-theme unpinned in all repos, so we have full central control over the base theme if there are pydata-sphinx-theme bugs.

[consideRatio]

@manics I think you are right!

See the difference between the preview robots.txt https://zero-to-jupyterhub.org.readthedocs.build/robots.txt vs the production robots.txt https://z2jh.jupyter.org/robots.txt. robots.txt is blocked in the preview setting, but not for RTD production pages not meant for preview.

It makes sense, they should stop scrapers etc from finding preview pages. I think things may work if we merge this :)

[choldgraf]

@manics if we define a jupyterhub theme we could also consider defining some extensions that are automatically installed in all of our repositories (e.g. sphinx-copybutton) or some theme styles to standardize (e.g. the primary color could be jupyter orange). Another option would be to use a single-page theme like the Sphinx Book Theme and then define a topbar in the jupyterhub theme that is shared between all projects. This is what the Dask project does as well as the 2i2c docs. I actually have a PR open in the book theme to make this easier to do, would love comments if you think it'd be helpful: executablebooks/sphinx-book-theme#559

[consideRatio]

Things seem to work overall, there is an issue with a reference to favicon.ico but other than that, things work!