nixos-render-docs: Display relevant source for errors

Most of the error messages in `nixos-render-docs` produce line numbers,
which would be really helpful if the sources being rendered weren't
often generated from `.nix` files themselves!

While the Python `markdown_it` library doesn't give us an obvious way to
track source provenance like this (i.e., mapping source spans in the
generated files to source spans in the input files), we can make the
errors a lot clearer by displaying the relevant lines from the source.

Below, an error message caused by a missing `:::` is made obvious by
displaying the span that composes the block being rendered, rather than
pointing only at the offending title.

Before:

```
caused by:
        unexpected non-title heading in example in line 87
```

After:

```
caused by:
unexpected non-title heading in `:::{.example}`; are you missing a `:::`?
Note: blocks like `:::{.example}` are only allowed to contain a single heading in order to simplify TOC generation.

`:::{.example}` block at lines 76-112:
  73 ┆ ```
  74 ┆
  75 ┆ ### Examples
  76 ┃ :::{.example}
  77 ┃ #### `lib.debug.traceValFn` usage example
  78 ┃
  79 ┃ ```nix
  80 ┃ traceValFn (v: "mystring ${v}") "foo"
  81 ┃ trace: mystring foo
  82 ┃ => "foo"
  83 ┃ ```
  84 ┃
  85 ┃ Located at [lib/debug.nix:106](https://github.com/NixOS/nixpkgs/blob/master/lib/debug.nix#L106) in `<nixpkgs>`.
     ...
 103 ┃ ### Examples
 104 ┃ :::{.example}
 105 ┃ #### `lib.debug.traceVal` usage example
 106 ┃
 107 ┃ ```nix
 108 ┃ traceVal 42
 109 ┃ # trace: 42
 110 ┃ => 42
 111 ┃ ```
 112 ┃
 113 ┆ :::
 114 ┆
 115 ┆ Located at [lib/debug.nix:135](https://github.com/NixOS/nixpkgs/blob/master/lib/debug.nix#L135) in `<nixpkgs>`.

Unexpected heading at line 87:
  84 ┆
  85 ┆ Located at [lib/debug.nix:106](https://github.com/NixOS/nixpkgs/blob/master/lib/debug.nix#L106) in `<nixpkgs>`.
  86 ┆
  87 ┃ ## `lib.debug.traceVal` {#function-library-lib.debug.traceVal}
  88 ┆
  89 ┆ Trace the supplied value and return it.
  90 ┆
```

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