Prepare for upcoming *.txt -> *.adoc bulk rename

[dscho]

There is a proposed patch series to rename the *.txt files in Documentation/ to *.adoc.

Where *.txt files are available, people frequently run various commands to parse and consume them, including tooling for static websites as this one. Which means that such a rename breaks that tooling.

I agree that this would have been a nice feature to add at the beginning of the development of the documentation, but I fear that it is too late to make an incompatible change now. However, as opposed to increasing Git's safety stance, the convenience of a few who want to have syntax highlighting in editors based on the file extension apparently is important enough to merit just such an incompatible change.

This means that script/update-docs.rb will need to be adapted once the tooling here breaks. Or before it. If someone volunteers.