README: update build instructions & prerequisites

[benglin]

Purpose

This PR updates the DynamoRevit README.md to reflect current build requirements and modernize the documentation for developers. The changes improve clarity, accuracy, and usability of the build instructions by incorporating information from the current codebase configuration and removing outdated references.

• Updated Prerequisites: Modernized software requirements (Revit 2026/2027, Visual Studio 2022, .NET 8/10 SDKs) with clear version specifications and download links
• Improved Build Instructions: Made environment variables optional, clarified platform selection (NET80/NET100), and restructured DLL handling as conditional rather than mandatory
• Enhanced Documentation Structure: Added dedicated prerequisites section, improved step numbering, moved technical details to contextual blockquotes, and clearly marked optional steps
• Technical Accuracy: Verified and corrected DLL lookup order based on actual props files, updated all paths to current build structure, and modernized version references
• Removed Outdated Information: Eliminated redundant instructions, made manual DLL copying conditional, and removed references to outdated AnyCPU/ProgramData paths

These changes make the README more accurate, easier to follow, and better aligned with the current codebase configuration, reducing setup friction for new developers.

Declarations

Check these if you believe they are true

• [x] The code base is in a better state after this PR
• [x] Is documented according to the standards
• [x] The level of testing this PR includes is appropriate
• [x] User facing strings, if any, are extracted into *.resx files
• [ ] Snapshot of UI changes, if any.

Reviewers

(Please assign appropriate reviewer from the Dynamo team)

FYIs

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