Make title levels consistent in manual.rst
Make title levels consistent in manual.rst

Ensure that the same marks are used for the same title level throughout
the manual as otherwise it will not build with newer docutils.

The level underlines in use are:

    level 1: ============
    level 2: ------------
    level 3: ~~~~~~~~~~~~
    level 4: ************
    level 5: """"""""""""

rst2pdf/rst2pdf,rst2pdf/rst2pdf