Enhanced documentation for the EnvelopeGenerator system:
- Added purpose section describing its functionality as a digital document signing system.
- Documented solution structure, listing projects, frameworks, and purposes.
- Highlighted key files in `ReceiverUI` and `API` with their roles.
- Explained `AnnotationDto` coordinate system and unit conversions.
- Detailed the signing flow in `ReceiverUI`, including page load, signature popup, and export steps.
- Documented JavaScript checkbox overlay logic for annotations.
- Provided a breakdown of `StampSignaturesOnPdf` using iText7.
- Described `BuildFreshBaseReport` behavior in real and dev modes.
- Listed NuGet packages with versions and purposes.
- Added "Mistakes History" to prevent repeated errors.
- Clarified why DevExpress X.509 signing article does not apply.
- Summarized development sessions in a change log.
