Ability to reference other files in .md tests

this allow:

• more testing of includes
• first step towards actually being able to verify ligo CLI output within the doc
• testing META ligo (testing framework)

We also now find .md files instead of relying on an hardcoded list.

Reminder on how to use .md checker:

<Syntax syntax="<syntax>">
```<syntax> <arguments>

<code>

```
</Syntax>

<arguments>:
 (*skip testing for this code block*)
 | skip
 (*will append code with code of all code blocks of the same group*)
 | group=<group_name>
 (*interpret code block group as test file (e.g. `ligo test <code_block> test`), so you must have a 'test entrypoint'*)
 | test-ligo group=<group_name>

relates to #574