Add sphinx copybutton extension

[wxtim]

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.

Add a copy button to code snippets.

I've wanted to do this for ages, but last time I looked this extension didn't exist.

Built at https://████████████████/doc/cylc/8.5.2.dev/html/tutorial/scheduling/graphing.html#example

[oliver-sanders]

(please test this out with diff code-blocks)

[MetRonnie]

Works well apart from diff blocks:

What ends up copied:

 [scheduler]
    allow implicit tasks = True
 [scheduling]
    cycling mode = integer
    initial cycle point = 1
    [[graph]]
-        R1 = """
        P1 = """
            buy_ingredients => make_dough
            pre_heat_oven & make_dough => bake_bread
            bake_bread => sell_bread & clean_oven
        """

However I'm not sure if there is a good need to copy diff blocks.

Also, it might be tricky to solve as there is something funky going on with diff blocks when you try to select the text:

[oliver-sanders]

Also, it might be tricky to solve as there is something funky going on with diff blocks:

Diff blocks have some CSS rules to make it easier for users to copy paste:

https://cylc.github.io/cylc-sphinx-extensions/extensions/cylc.sphinx_ext.diff_selection.html

However, this is CSS only, just display, nothing is different about these blocks otherwise.

Suggest turning the copy feature off for diff blocks to avoid user pain in training sessions? Or finding a way to hack it?

[MetRonnie]

(Test failures will be fixed by #866)