hdf5doc

_ Changed line 172: added anchor for the Intro xref. 
Tested: Firefox

_ Changed a link so that its target is now 
ObjectHeaderMessages which is section "Disk Format: 
Level 2A2 - Data Object Header Messages." Target of 
link used to be the non-existent SharedMessage 
bookmark. 
Tested: Firefox

_ Changed the target of the link to the HDF5 User's 
Guide in the "This Document" section to 
"UG/index.html" from "H5.user.html". 
Tested: Firefox

_ Changed link from wiki.hdfgroup.org to 
http://www.hdfgroup.org/services/contributions.html. 
Removed footnote.
Tested: Firefox