check attributes usage from markdown or RST

[docsAsCode]

adoc can use attributes to replace some values in a document. The attribute must be declared in the header as :myattribute: value and can be used in the body as {myattribute}.

This issue must check :

• [x] we can declare & use a custom attribute in markdown
• [ ] we can declare & use a custom attribute in RST

[jerCarre]

---
title: test attribute
myattribute: value
---

## first part

in this part we speak about {myattribute}

The attribute declaration is well transmitted into adoc file but the left brace is escaped by converter on attribute usage. => issue on kramdown-asciidoc

[docsAsCode]

fixed by code for MD

[docsAsCode]

For RST, must use substitution directive, but ... bug with pandoc : https://github.com/jgm/pandoc/issues/4160