Document the flex-context modal dialogue

[joshuaunity]

In https://flexmeasures.readthedocs.io/stable/views/asset-data.html we should add a quick section about this new dialogue.

It should explain what the flex-context is (link to its page), and the show the dialogue.

Also, the i-icon next to "Edit Asset's FlexContext" has too little content - add some. And it is not displaying when I hover it (on Firefox). This also happens on the graph dialogue for me, by the way.