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/languages-frameworks/vim.section.md

@@ -12,7 +12,7 @@ At the moment we support three different methods for managing plugins:
 - Pathogen
 - vim-plug
 
-## Custom configuration
+## Custom configuration {#custom-configuration}
 
 Adding custom .vimrc lines can be done using the following code:
 
@@ -56,7 +56,7 @@ neovim-qt.override {
 }
 ```
 
-## Managing plugins with Vim packages
+## Managing plugins with Vim packages {#managing-plugins-with-vim-packages}
 
 To store you plugins in Vim packages (the native Vim plugin manager, see `:help packages`) the following example can be used:
 
@@ -116,7 +116,7 @@ The resulting package can be added to `packageOverrides` in `~/.nixpkgs/config.n
 
 After that you can install your special grafted `myVim` or `myNeovim` packages.
 
-### What if your favourite Vim plugin isn't already packaged?
+### What if your favourite Vim plugin isn’t already packaged? {#what-if-your-favourite-vim-plugin-isnt-already-packaged}
 
 If one of your favourite plugins isn't packaged, you can package it yourself:
 
@@ -154,7 +154,7 @@ in
 }
 ```
 
-## Managing plugins with vim-plug
+## Managing plugins with vim-plug {#managing-plugins-with-vim-plug}
 
 To use [vim-plug](https://github.com/junegunn/vim-plug) to manage your Vim
 plugins the following example can be used:
@@ -183,14 +183,14 @@ neovim.override {
 }
 ```
 
-## Managing plugins with VAM
+## Managing plugins with VAM {#managing-plugins-with-vam}
 
-### Handling dependencies of Vim plugins
+### Handling dependencies of Vim plugins {#handling-dependencies-of-vim-plugins}
 
 VAM introduced .json files supporting dependencies without versioning
 assuming that "using latest version" is ok most of the time.
 
-### Example
+### Example {#example}
 
 First create a vim-scripts file having one plugin name per line. Example:
 
@@ -280,7 +280,7 @@ Sample output2:
 ]
 ```
 
-## Adding new plugins to nixpkgs
+## Adding new plugins to nixpkgs {#adding-new-plugins-to-nixpkgs}
 
 Nix expressions for Vim plugins are stored in [pkgs/misc/vim-plugins](/pkgs/misc/vim-plugins). For the vast majority of plugins, Nix expressions are automatically generated by running [`./update.py`](/pkgs/misc/vim-plugins/update.py). This creates a [generated.nix](/pkgs/misc/vim-plugins/generated.nix) file based on the plugins listed in [vim-plugin-names](/pkgs/misc/vim-plugins/vim-plugin-names). Plugins are listed in alphabetical order in `vim-plugin-names` using the format `[github username]/[repository]`. For example https://github.com/scrooloose/nerdtree becomes `scrooloose/nerdtree`.
 
@@ -298,7 +298,7 @@ To add a new plugin, run `./update.py --add "[owner]/[name]"`. **NOTE**: This sc
 
 Finally, there are some plugins that are also packaged in nodePackages because they have Javascript-related build steps, such as running webpack. Those plugins are not listed in `vim-plugin-names` or managed by `update.py` at all, and are included separately in `overrides.nix`. Currently, all these plugins are related to the `coc.nvim` ecosystem of Language Server Protocol integration with vim/neovim.
 
-## Updating plugins in nixpkgs
+## Updating plugins in nixpkgs {#updating-plugins-in-nixpkgs}
 
 Run the update script with a GitHub API token that has at least `public_repo` access. Running the script without the token is likely to result in rate-limiting (429 errors). For steps on creating an API token, please refer to [GitHub's token documentation](https://docs.github.com/en/free-pro-team@latest/github/authenticating-to-github/creating-a-personal-access-token).
 
@@ -312,7 +312,7 @@ Alternatively, set the number of processes to a lower count to avoid rate-limiti
 ./pkgs/misc/vim-plugins/update.py --proc 1
 ```
 
-## Important repositories
+## Important repositories {#important-repositories}
 
 - [vim-pi](https://bitbucket.org/vimcommunity/vim-pi) is a plugin repository
   from VAM plugin manager meant to be used by others as well used by