Doc: Avoid auto-linking the string 'WebEngine' to the QML type

Remove the previous workaround (internal \externalpage command) that
was used for this purpose, and replace it with a \QWE macro that
expands to the string 'Qt \WebEngine'. The backslash in the expanded
string instructs QDoc not to attempt auto-linking the word.

Change-Id: If4e1c95423fa07479b1af055e4760a890c0ac667
Reviewed-by: Paul Wicking <paul.wicking@qt.io>