Add versionadded, changed, deprecated admonitions

[choldgraf]

This adds examples of the three "version admonitions", which seem to be special cases of admonitions. I couldn't find them in the sphinx themes gallery but have seen many docs using them.

Example of what they look like in the pydata theme:

[12rambau]

Sorry I've approved this PR thinking it was living in pydatasphinx theme. I eon't have any role here, don't take my review into consideration

[shirou]

confirmed Insegel theme. Thank you!