PDF document signing - greek tutorial - better SVG image parsing

Added

• new methods to allow signing PDF documents: link to docs
• support for colors defined with the rgb() syntax in SVG images - cf. #480
• New translation of the tutorial in Ελληνικά, thanks to @sokratisvas

Changed

• an /ID is now inserted in the trailer dictionary of all PDF documents generated.
  This ID can be controlled through the new file_id() method.
• the svg.path package was added as a dependency to better parse SVG images

Fixed

• font_stretching doesn't make text spill out of multi_cell() and write() boundaries anymore, thanks to @gmischler
• local_context() now always restores the correct font settings after finishing, thanks to @gmischler
• properly parsing single-digits arguments in SVG paths - cf. #450
• document outline encoding: it was found to be broken when using a thai font - cf. #458