Plugin manual - Added CSS, improved HTML format

- Added a CSS style sheet to plugin manual folder
- Revised HTML formatting, by removing (almost) all hard-coded formats / styles and implementing them on the CSS side
- Added some styles to improve property table readability
- Added a logo (via CSS) and a footer to each page
- No change to the code logic.
Fix crash when adding text to a newly created SimpleText element.

When some text is typed for a newly created SimpleText text item (staff, system, ...), the program crashes.

Added a test to check the SimpleTex::_text list is not empty before adding to it.
fix #19443
fix typo

for Plugins documentation
fixing some dynamics

using "&dynamicZ;" for 'z' rather than "&dynamicNiente;". The latter is
not available in Emmentaler
For Windows Qt 5.2 is now the minimum

Since commit ac1a3fbb58 Qt 5.2 is the minimum requiremennt for Windows
Fix #24409: crash when adding textual elements

Adds another test for TLine::_text being empty or not.
fix for marker
Merge pull request #669 from Jojo-Schmitz/Qt-5.2-for-Windows

For Windows Qt 5.2 is now the minimum
merge
Merge branch 'master' of github.com:musescore/MuseScore
Merge pull request #666 from mgavioli/Fix_crash_on_adding_to_SimpleText

Fix #24409 : crash when creating a new SimpleText element or adidgin text to an empty one
Merge pull request #664 from mgavioli/Plugin_manual_css_and_html_styles

Plugin manual - Added CSS, improved HTML format