Allow builds without an internet connection

[lurch]

When building the documentation site locally (e.g. for previewing edits), https://github.com/raspberrypi/documentation/blob/develop/scripts/create_htaccess.py currently connects to https://rptl-datasheets.s3.eu-west-1.amazonaws.com to check which datasheets URLs are valid. If you're e.g. working on a laptop without an internet connection, this causes the whole build to fail! :facepalm: I should probably add an environment variable or something (or maybe just a try ... catch block) so that local-builds still work even when offline.