Make qtassistant a compile time option

[peterfpeterson]

This removes the pointer to the qtassistant wrapper as dependent on the variable QT_DOCS. The default behavior is to keep the current functionality.

Description of work

Summary of work

Refs #37248 and EWM6548

Further detail of work

To test:

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[github-actions]

👋 Hi, @peterfpeterson,

Conflicts have been detected against the base branch. Please rebase your branch against the base branch.