Fix table of contents sidebar overlapping with EthicalAds placement

[laymonage]

Fixes #293.

Demo

Before

https://github.com/user-attachments/assets/f472038f-ee66-4867-9452-0632e0079ab1

After

https://github.com/user-attachments/assets/75ff9f11-6c1c-44fa-b986-f18b664c8f9e

Issue recap

EthicalAds placement is done with the following CSS:

[data-ea-style=stickybox].loaded {
    position: fixed;
    bottom: 20px;
    right: 20px;
    z-index: 100;
}

Meanwhile, our sidebars use Bootstrap's sticky-top class that applies the following:

@supports (position: sticky) {
    .sticky-top {
        position: sticky;
        top: 0;
        z-index: 1020;
    }
}

There are a few ways we could go about this.

1. Change the z-index of the sticky-top class from Bootstrap, that we use for both sidebars.
2. Add styles for [data-ea-style=stickybox].loaded. This seems brittle.
3. Use our own CSS to apply the stickiness. For example, with the following CSS:

   .sticky {
    position: sticky;
    top: 0;
    z-index: 20;
   }

   And then use it in the following places: https://github.com/wagtail/sphinx-wagtail-theme/blob/25ccf6408139f9db09486c780ec86c1455f20d62/sphinx_wagtail_theme/layout.html#L114 https://github.com/wagtail/sphinx-wagtail-theme/blob/25ccf6408139f9db09486c780ec86c1455f20d62/sphinx_wagtail_theme/layout.html#L156

This PR uses approach 1. I changed all of the z-index values and not just the sticky one, because Bootstrap's docs says the following:

We don’t encourage customization of these individual values; should you change one, you likely need to change them all.

I'm not sure of the implications this may have though, but from what I can tell our theme doesn't use dropdowns, fixed, modals, popovers, or tooltips.