Update doc about plutosvg

[RT2Code]

This PR microsoft/vcpkg#43567 added plutovg/plutosvg ports to vcpkg and replaced the lunasvg feature of the imgui port with plutosvg.

[ocornut]

Thanks! This is good news. May i suggest we kept an explicit blurb for manual compiling? But it may be moved after the vcpkg suggestion and compacted into less lines (e.g. "Compile all source files in plutovg/source/*.c + Add include directory: plutovg/include + plutovg/stb" etc.).

[RT2Code]

Something like this?

Using plutosvg (and plutovg)

• Add #define IMGUI_ENABLE_FREETYPE_PLUTOSVG in your imconfig.h.
• Get latest plutosvg binaries or build yourself. Under Windows you may use vcpkg with: vcpkg install plutosvg --triplet=x64-windows. Alternatively, if you build imgui from vcpkg, you just need to enable the plutosvg feature: vcpkg install imgui[plutosvg] --triplet=x64-windows
• If you prefer to build plutosvg manually:
  • Compilation hints for plutovg: Compile all source files in plutovg/source/*.c + Add include directory: plutovg/include + plutovg/stb
  • Compilation hints for plutosvg: Compile plutosvg/source/plutosvg.c + Add include directory: plutosvg/source + Add define: PLUTOSVG_HAS_FREETYPE + Link with: plutovg, freetype

I removed the build hints because I just try to follow the current documentation style as closely as possible, and as there were none for lunasvg, I assumed you deemed them unimportant if it was provided by vcpkg. Maybe it's best to leave the build instructions as it was before for readability.