docs: rewrite plugin install/troubleshooting guide

[DmitryLambin]

Closes #6078.

Summary

Rewrites the two plugin documentation pages that new users land on when adding a plugin to MeshLib or MeshInspector:

• HowtoAddPluginOverview.dox — entry point, install + troubleshooting + uninstall
• ExamplePluginOverview.dox — build steps for the example plugin per platform

What's new

HowtoAddPluginOverview.dox

• Trust & safety section. Plugins are native code loaded into the host process with full user privileges; the prior text told users to sudo cp arbitrary .dylib files into /Applications/MeshInspector.app without a single word about source verification.
• Per-platform install sections at parity. Previously only Windows had concrete steps; macOS was missing entirely and Linux was implicit.
• macOS App Bundle Protection explained, with the user-writable clone (recommended) and sudo-into-/Applications/... (advanced) presented in that order. The sudo path now carries an explicit warning about breaking the bundle's code signature and what to do if Gatekeeper blocks the next launch (codesign --force --sign -).
• dyld / RPATH / SONAME troubleshooting. CMake INSTALL_RPATH + BUILD_WITH_INSTALL_RPATH ON with rationale, otool -L audit, install_name_tool -change workflow with explicit "read your own deps first" guidance (so users do not blindly paste example versions), libcpr.1.dylib symlink case.
• Verifying the install with concrete log paths per platform and a matrix of common failure modes mapped to fixes.
• Uninstall section.
• Windows: clarified "Run as administrator", added CRT debug/release mismatch and Windows Defender / SmartScreen notes.
• Linux: dpkg -L meshinspector hint so users on tarball / AppImage / from-source can find their own paths.
• "What a plugin distribution looks like" so users receiving a precompiled plugin from a third party have a starting point — previously the docs assumed "I just built it".
• Reformatted the Version Compatibility section (the previous nested bullets did not render).

ExamplePluginOverview.dox

• Added Linux and macOS Usage sections with CMake invocations against MeshLib.framework / MeshLib_DIR, plus an otool -L verification step.
• macOS deploy step now recommends the cloned-folder approach; the system-wide install is referenced to HowtoAddPluginOverview rather than duplicated, to keep the code-signature warning in one place.
• "After install — quick sanity check" with the per-platform log paths.

Test plan

• Doxygen build of the docs renders without warnings on the new sections (specifically the \warning / \note blocks and \section anchors used for cross-references).
• Visual check at https://meshinspector.github.io/MeshLib/dev/html/HowtoAddPluginOverview.html after CI publish — TOC shows the new sections, code blocks render with correct syntax highlighting, cross-references between the two pages resolve.
• Spot-check that no \ref anchors point at sections I renamed (only HowtoAddPluginOverview_VersionCompatibility was added; cross-page references kept).

I have not changed code or build logic, so existing CI should pass unchanged.