feat: add two "guard" blocks that surround the digest

[NicolasIRAGNE]

Not sure how to call these but, in the likes of e.g. gitignore.io we could have a little prefix and suffix for the output.

Something like:

# Generated using https://gitingest.com/cyclotruc/gitingest (commit efe0000)

=====
blablabla
====

# End of generated file (commit efe0000)

For non-repo ingests, just use the name of the directory I think.