Add project: mkdocs-snippets

[SaltyAimbOtter]

What kind of change does this PR introduce?

• Add a project
• Update a project
• Remove a project
• Add or update a category
• Change configuration
• Documentation
• Other, please describe:

Description:

I added the snippet plugin I wrote a while back for another project. It has a few cool features and I hope more people will reuse it if it is listed here.

Checklist:

• I have read the CONTRIBUTING guidelines.
• I have not modified the README.md file. Projects are only supposed to be added or updated within the projects.yaml file since the README.md file is automatically generated.

[SaltyAimbOtter]

CI fail cause:

 mkdocs-snippets:
     'plugin' label should be present if and only if 'mkdocs_plugin:'    

What's the matter the mkdocs_plugin? It is not documented in the CONTRIBUTING.md

[pawamoy]

@SaltyAimbOtter if you look at other projects above, you'll see they specify a mkdocs_plugin key-value pair. It's the name of the plugin, as specified in mkdocs.yml under the plugins configuration item. Sorry the contribution guide doesn't mention it, I believe it's the original CONTRIBUTING file from the repo we forked, and we didn't update it with our customizations.