Enforce file extensions

[siddhantk232]

The sitemap works with documents with no extensions as well. This is a bug, we want the user to explicitly mention file extensions.

Current behaviour:

Below code works and assumes that the document features/design is a .ftd file.

-- fastn.sitemap:

- Design: /design/
  document: features/design

Expected behaviour:

The above code should error (and maybe suggest that the user is missing the file extension).

Correct code:

-- fastn.sitemap:

- Design: /design/
  document: features/design.ftd

[amitu]

A small correction in your code snippets, we do not need -- end: fastn.sitemap as fastn.sitemap does not expect "children".

[siddhantk232]

A small correction in your code snippets, we do not need -- end: fastn.sitemap as fastn.sitemap does not expect "children".

Thanks! I've updated the comment :+1: