[docs] Enable some new features (#2623)

* [docs] Enable a bunch of markdown extensions

* details makes admonitions collapsible and when started with ???
  instead of !!! they'll be collpased by default
* highlights are updated to include linenums by default but with a style
  that doesn't result in the linenums to be copy-pasted when selecting
  and pasting. This makes it possible to directly link to a specific
  line in the documentation instead of just the general page
* caret, mark and tilde make it possible to highlight text and have
  super/subscripts
* keys turns combos like `++ctrl+alt+del++` into HTML key elements
  showing a keyboard combination to press
* tabbed makes it possible to have tabs within a document. Right now we
  have different sections sometimes to show the config for nginx, apache
  and Caddy, which can be turned into tabs instead and which tab is
  picked will get remebered
* smartsymbols turns certain things, like `(c)` in the right symbol ©

* [docs] Upgrade all the python dependencies

* [docs] Explain how to update conda deps

CONTRIBUTING.md

@@ -110,6 +110,8 @@ When adding a new page, you need to include it in the [`mkdocs.yml`](mkdocs.yml)
 
 If you don't use Conda, you can read the `docs/environment.yml` to see which dependencies are required and `pip install` them manually. It's advisable to do this in a virtual environment, which you can create with something like `python3 -m venv /path-to/store-the-venv`. You can then call `/path-to/store-the-venv/bin/pip`, `/path-to/store-the-venv/bin/mkdocs` etc.
 
+In order to upgrade dependencies, use `conda update --update-all` in the activated environment. You can then update the `environment.yml` with `conda env export --from-history -f ./docs/environment.yml`, though you'll need to fix the `channels`. Beware that `conda env export` will also drop the `pip` dependencies, so make sure to add those back.
+
 ## Development
 
 ### Golang forking quirks