Rename Measuring intro file to get better URLs

[spier]

gitbook generates the URLs for the markdown files by using the filename itself. When the same filename exists multiple times in different directories, it applies some custom logic to prevent collisions. Not sure why.

You can see this in effect in the current "What, When and How to Measure" section, as the corresponding markdown file is called "introduction.md" which already exists, you guessed it, in the "Introduction" chapter :)

This leads to unfortunate URLs like this:

https://innersourcecommons.gitbook.io/managing-innersource-projects/introduction-1
https://innersourcecommons.gitbook.io/managing-innersource-projects/introduction-1/goals

How to fix

This PR fixes this by renaming measuring/introduction.md to measuring/measuring.md.

This will also have the extra benefit of generating a nicer landing page for the whole chapter, which will then be https://innersourcecommons.gitbook.io/managing-innersource-projects/measuring

Screenshots

[spier]

This change as a slightly more "breaking" nature to it, therefore I will leave this here for somebody to review :)

[spier]

Lovely. The chapter is now live at the modified URL at https://innersourcecommons.gitbook.io/managing-innersource-projects/measuring