docs: fix file_link on Read the Docs

In RtD mkdocs build absolute path is supplied - make it relative when
that happens.
5 jobs for fix-doc-file-link in 2 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed flake8 #525610

00:01:02

passed integration #525613

00:01:45

passed pylint #525611

00:01:52

passed self #525614

00:02:18

passed unit #525612

00:01:42