ExternalData: Format module documentation

Manually revise the .rst format of the documentation.  Use inline
literal quotes appropriately in paragraph text.  Move the :: literal
block openers to the end of the preceding paragraphs.  Convert the
command signature documentation and examples to cmake code-block
directives.
ExternalData: Show example network path URL template

In the ExternalData_URL_TEMPLATES example, add an entry showing how to
use the "file://" protocol with a network filesystem path.
Merge topic 'doc-ExternalData-network-path' into next

f3883f3 ExternalData: Show example network path URL template
ff6818b ExternalData: Format module documentation