Placeholder for internal usage: create Placeholder documentation

[nochso]

Now that all placeholders must specify their supported options, it should be easy to automatically create documentation for all placeholders.

See https://github.com/nochso/writeme/commit/54ac048c3018f9797c215438cb9c19846b23d4bb

@placeholder.doc@ should output:

• All available placeholder identifiers by looking for classes that implement the Placeholder interface
  • PHPdoc of the placeholder class
• Their supported options
  • Frontmatter key/path
  • Description
  • Default value

[nochso]

Closed by https://github.com/nochso/writeme/commit/506e2f44426a2f1a12a3e6df30cf7236b8c1ec4c