Remove special styling of sidebar heading

[timhoffm]

PR Summary

The previous styling looked a bit dated and is harder to read.

before / after:

[story645]

I like but does there even need to be "table of content" or can that just be in a div label or something for screen readers?

[tacaswell]

Should we leave styling to set the heading color to the blue we use else where?

[timhoffm]

@tacaswell Blue color is currently only used for <h1> headings. In particular I would not color the sidebar heading. Quite the opposite, I was considering also removing the color from the sidebar links, because the local toc should be visually restrained.

@story645 IMHO there should be something. I like the pandas-way:

Anyway, changing the words is a bit more complicated. Currently, they come directly from Sphinx. I suspect, we'd have to vendor our own version of the sidebar template. I'm not sure how far I want to push style changes on the current theme given that we might consider switching to the pydata theme. I'm just picking the lowest hanging fruits with a high annoyance/effort ratio.

[jklymak]

Lets merge this, and @story645 if you want to have a follow-on issue, that seems fair. I agree its a bit funny to have such an obvious heading, but...

[timhoffm]

... Sphinx wants it that explicit. The PR doesn't change the wording. We can style it white on white 🙃 .