Expose property for relative source path

[justinp]

I'm using paradox from a Bitbucket repository and I'd like to make source links the way they exist for GitHub. Rather than making a specific property, I decided to try to piece the URL together from parts. The only part that I can't find a way to access is the relative path from the root of the repository to the markdown source file.

This change adds relative to Page so that, for example $page.self.relative$ can be used.

[lightbend-cla-validator]

At least one commit author (jpatterson@affinipay.com) is not linked to a user. See https://help.github.com/en/articles/why-are-my-commits-linked-to-the-wrong-user#commits-are-not-linked-to-any-user

[lightbend-cla-validator]

Hi @justinp,

Thank you for your contribution! We really value the time you've taken to put this together.

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it.
Please review the new CLA and sign it before we proceed with reviewing this pull request:

https://www.lightbend.com/contribute/cla

[pvlugter]

Seems reasonable to me. Thanks.

[pvlugter]

Looks like the CLA check is failing, as the CLA has been updated.

[justinp]

Thanks.

I noticed the CLA thing, but I submitted it right after I opened this PR and again just now, and it still seems to appear stuck.

[pvlugter]

Re-triggered the CLA check with reopening the PR. All good. Thanks.

[justinp]

Thanks for merging it!

What's the process I need to go through to get an officially published artifact I can use?

[pvlugter]

Released paradox 0.10.3 with this change.