Remove ApplicationHtmlDocGenerator

Summary

• Remove ApplicationHtmlDocGenerator
• Replace the "Documentation" tab in otbgui with a standard Help menu (click or press F1) that links to the online CookBook
• Fix qt layout problem in the footer

TODO on server side:

• Replace https://www.orfeo-toolbox.org/Applications/ with a link to https://www.orfeo-toolbox.org/CookBook/Applications.html

Rationale

Part of story #1679

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 👍 votes from core developers, no 👎 vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement