Make the TFile layout doc visible in Ref Guide (#7891)

This PR makes the TFile Layout documentation visible in
the Doxygen ROOT reference Guide.

- The .txt file have been turned into markdown
- Doxyfile have io/doc/TFile folder as input
- References/links between the pages have been setup
- The \tableofcontents Doxygen functionality  has been turned ON
- Some typos have been fixed

Still two links need to be resolved:
  http://root.cern.ch/root/RootDoc.html
  http://root.cern.ch/root/htmldoc/examples/Version302.news.html