Options for doc-comment rendering: markdown or `<pre>` block

Options for doc-comment rendering: markdown or `<pre>` block

Support of `<pre>` blocks needed, e.g., to create documentation from [zig stdlib](https://ziglang.org/documentation/master/std/) or other codebase, where doc comments are not markdown formatted, but rather are plain text.

_Originally posted by @insolor in https://github.com/insolor/mkdocstrings-zig/discussions/1_