fix: display math characters in the header navbar

[12rambau]

Fix #226
In the header toctree function I rewrote the content of the title using a span if a math node is included.

to be interpreted inline math needs to be in ["(", ")"] characters (and compatible with mathJax of course).

Worked on my test documentation with "About :math:\alpha":

[choldgraf]

Just a quick comment to clarify what the function does and when it is meant to be used. Did I get that correct? Other than this it seems good to me!

So this is only applied to the table of contents, not to the in-page header (which I guess has math displayed properly)

[12rambau]

So this is only applied to the table of contents, not to the in-page header (which I guess has math displayed properly)

from what I understand, matJax is simply looking for \( and \) to replace it by inline math. in the other sidebars, we use the node itself so it works but in the header we wrote down the HTML from the tilte so we only get the text (avoiding the processing of the docutils node).

I noticed other places where mathJax is actually not running as expected (prev-next button, toctree in articles) but as it's not working with sphinx default theme that's not for us.

PS: waiting for 0.11 to be released before merging