feat: enhance blog-aware rules to handle more use cases (blog, docs...)

[frouo]

Purpose

Allow to customize the YYYY-MM-DD rule for given folder.

Thus allowing specified folder to have custom rule to sort content:

pages-mardown/
├ docs/
   ├ feat-A-doc1.md
   ├ feat-A-doc2.md
   ├ feat-B-doc1.md
   ├ feat-B-doc2.md
   ├ feat-B-doc3.md
``

## how to?

Please take example to the function `include` in the NextMarkdownConfig type, where `include` has a default behavior that can be overridden.

[frouo]

Here are the rules we should go for:

• rule 1: we keep the prefix _ rule : files starting with _ will be ignored (ie. next-markdown won't create a path for it). It allows to easily publish / unpublish a page
• rule 2: in a given folder, every files (except index.md) will be listed in the index.md rendered page.
• rule 3: if any, text within the brackets at the beginning of the file (but after _ see rule 1) will be ignored (eg: file [feat1]hello_world.md will create /hello_world)

Note: having bracket is totally optional (cf. rule 2)

Examples, resulting to the same behavior

	brackets

The index.md rendered page will received props:

props: {
  ...
  posts: [
    {
      ...
      nextmd: [docs, get-started]
    },
    {
      ...
      nextmd: [docs, blog-aware]
    },
    {
      ...
      nextmd: [docs, dynamic-routes]
    },
    {
      ...
      nextmd: [docs, table-of-contents]
    },
    {
      ...
      nextmd: [docs, examples]
    },
    {
      ...
      nextmd: [docs, contributing]
    },
  ]

What if I don't want a file to be listed?

Answer: create a folder; cf. rule 2: only files will be listed in the index.md rendered page.

In the following example, /docs/contribution will not be listed in the rendering of /docs