Document .statiq files

[daveaglick]

See https://github.com/statiqdev/Statiq.Web/issues/967

• Add a separate page explaining .statiq files and the use cases
• Explain that .statiq extension is dropped
• Explaining that .statiq files are always read, even if they start with an underscore and add a note to https://www.statiq.dev/web/content-and-data/ about it
• Show an example of using ContentType and MediaType to treat the .statiq file as something different