ci: Ignore Terraform generated docs in the linter (#2)

Terraform provider documentation is automatically generated and can fail
some markdown lint checks (like line length). 

This change removes the default configuration setting (lint all Markdown 
files in the repository) and adds the `docs` path to the ignore field which 
is where Terraform generated docs live.

All other markdown files should be linted.

.github/workflows/lint.yml

@@ -81,4 +81,4 @@ jobs:
         uses: articulate/actions-markdownlint@17b8abe7407cd17590c006ecc837c35e1ac3ed83 # v1.1.0
         with:
           config: .markdownlint.yml
-          files: '**/*.md'
+          ignore: docs