doc: prepare for commonmark

We are still using Pandoc’s Markdown parser, which differs from CommonMark spec slightly.

Notably:
- Line breaks in lists behave differently.
- Admonitions do not support the simpler syntax https://github.com/jgm/commonmark-hs/issues/75
- The auto_identifiers uses a different algorithm – I made the previous ones explicit.
- Languages (classes) of code blocks cannot contain whitespace so we have to use “pycon” alias instead of Python “console” as GitHub’s linguist

While at it, I also fixed the following issues:
- ShellSesssion was used
- Removed some pointless docbook tags.

doc/contributing/coding-conventions.chapter.md

@@ -462,9 +462,9 @@ Preferred source hash type is sha256. There are several ways to get it.
 
     For package updates it is enough to change one symbol to make hash fake. For new packages, you can use `lib.fakeSha256`, `lib.fakeSha512` or any other fake hash.
 
-    This is last resort method when reconstructing source URL is non-trivial and `nix-prefetch-url -A` isn't applicable (for example, [one of `kodi` dependencies](https://github.com/NixOS/nixpkgs/blob/d2ab091dd308b99e4912b805a5eb088dd536adb9/pkgs/applications/video/kodi/default.nix#L73")). The easiest way then would be replace hash with a fake one and rebuild. Nix build will fail and error message will contain desired hash.
+    This is last resort method when reconstructing source URL is non-trivial and `nix-prefetch-url -A` isn’t applicable (for example, [one of `kodi` dependencies](https://github.com/NixOS/nixpkgs/blob/d2ab091dd308b99e4912b805a5eb088dd536adb9/pkgs/applications/video/kodi/default.nix#L73)). The easiest way then would be replace hash with a fake one and rebuild. Nix build will fail and error message will contain desired hash.
 
-::: warning
+::: {.warning}
 This method has security problems. Check below for details.
 :::