Prevent double slash when using extra_files

[denisdefreyne]

Detailed description

The Troubleshooting page gives instructions for what to do when hidden files are ignored. The example given is as follows:

data_sources:
  -
    type: filesystem
    extra_files:
      - "/.well-known/**/*"
      - "/.nojekyll"

However, this example is not currently working as intended: the extra files end up with double slashes at the beginning (e.g. //.nojekyll).

This PR fixes that.

To do

• Tests

The troubleshooting documentation could also be updated, though if both variants (starting with slash and not starting with slash) are supported, the documentation is fine anyway.

Related issues

n/a