includes inside subdirectories not intuitive

[jsseidel]

Include blocks that exist in documents inside subdirectories only work if you use a path relative to the main project. This is confusing.

E.g. In project "FOO", create a directory called "components/bar". If you have multiple documents in "bar" that are included in some file inside "bar", you would do this:

```include
components/bar/myfile1.md
components/bar/myfile2.md
```

Probably it should work either way, but this might be more intuitive:

```include
myfile1.md
myfile2.md
```

In other words, mktechdocs should assume that included documents are (1) in the same directory as the document doing the including and then if that doesn't work, (2) relative to the project directory.