doc: qmllint warnings: remove extra list of sections + clean up

Remove the extra list of sections that are at the beginning of each qmllint warning, as they duplicate the table of content that is autogenerated and also lists the sections.
See also https://codereview.qt-project.org/c/qt/qtdeclarative/+/579065/comment/acc88c0e_5f6bee2a/.

Clean ups:

• remove extra list of sections
• Add "This warning category is called [...] by qmllint" right after the title and before the first \section1 for SEO, so that users can find the docs when they google the qmllint warnings.
• Replace all the "You can fix this warning by ..." with "To fix this warning, ..." to avoid using passive voice
• make sure that there are no section and titles that have the same name, as that might/will break linkage for sure.