Overwrite protection for static files generator

[fornib]

• [x] I have searched the issues (including closed ones) and believe that this is not a duplicate.
• [x] I have searched the documentation and believe that my question is not covered.
• [x] I am willing to lend a hand to help implement this feature.

Feature Request

When there is mixed content in the same directory, static files may overwrite generated pages without warning. This is undesirable, as you may not notice the lost information.

I propose to at least check writer._written_files before copying/linking static files and, if overwriting, just print a warning (without raising RuntimeError, as it would break existing websites).

[justinmayer]

Related issue from a decade ago: #446