help: update rendering.page

- add section Project settings and the rendering dialog
- change the bug note to tip
help: add a tip to "jump to an exact position" in movearoundtimeline.page
help: update welcomedialog.page
help: merge insertintimeline.page into usingclips.page

Migrate only the useful parts (not the obsolete "bug" note)
help: minor improvements in usingclips.page
help: merge saveproject.page and projectsettings.page as "workwithprojects.page"

Also update links that pointed to saveproject.page
help: minor improvements in workwithprojects.page
help: minor improvements in layers.page
help: merge makeselections.page and grouping.page as "selectiongrouping.page"

Also update links pointing to makeselections.page
help: minor improvements in selectiongrouping.page
help: update the Menu bar section in mainwindow.page
help: update The toolbars section in mainwindow.page
help: update the Ruler section in mainwindow.page
help: update the Timeline section in mainwindow.page
help: update the Playback toolbar section in mainwindow.page
help: update the Previewer section in mainwindow.page
help: add the Resizing components of the main window tip to mainwindow.page
help: turn the Media library section into Left pane section in mainwindow.page
help: add Middle pane section to mainwindow.page
help: add Timeline toolbar section to mainwindow.page