Documentation links to blocked images in the UK

[ZedThree]

Following the UK's implementation of the Online Safety Act, Imgur has blocked access to UK users. Because your docs link to images hosted on Imgur, these are not viewable in the UK, breaking your docs. For example, in the Variable docs:

ADIOS2/docs/user_guide/source/components/variable.rst

Line 91 in 83cb06a

.. image:: https://i.imgur.com/MKwNe5e.png

which to UK viewers appears as:

Because your docs are hosted on readthedocs.io, you could just put the images in your repo and have readthedocs host them.