Find a way to preserve links to source files in GitHub

[jcfreeman2]

I'm referring to Markdown snippets such as the following (in this case, in nwqueueadapters' README.md):

For an actual example, see [`FakeData.cpp`](./test/plugins/FakeData.cpp).

which work fine when the README.md is read directly on GitHub since the relative path to the package's source code is correct, but don't work when we've run the_final_markdown.sh to copy the Markdown files into the docs repo. In the above example it's because ./test/plugins/FakeData.cpp doesn't exist in the docs repo. Two possible solutions might be:

1. Have the_final_markdown.sh search for patterns of the form [<link label>](<obvious local source file>) and then provide a link to https://github.com/DUNE-DAQ/<package name>/blob/develop/<obvious local source file> (where we'll eventually need to get more sophisticated than always using "develop", but we'll cross that bridge when we get to it)
2. Have the_final_markdown.sh search for patterns of the form [<link label>](obvious local source file) and then copy over the source file into the actual docs/packages/<package name> directory