Remove unused video poster image

Add Derived Attributes documentation to MkDocs

- Add docs/concepts/derived-attributes.md with step-by-step tutorial
- Add video walkthrough support (docs/assets/videos/)
- Remove Doc/html folder (migrated to MkDocs markdown)
- Update examples/index.md with link to new location
- Update mkdocs.yml navigation
Improve documentation with fixes and expanded content

- Fix mermaid click links to use correct relative paths (../Class/ format)
- Expand Derivers section with OCL vs Delphi comparison, reverse derivation
- Add nested transactions documentation with diagram and examples
- Add OCL naming convention note (uppercase=class, lowercase=property)
- Improve EvaluateInPS and Variables examples in handle docs
- Standardize version number to 26.01.0

Author: bero

.gitattributes

@@ -1,6 +1,5 @@
 # Delphi/Windows project - use CRLF line endings
 * text=auto eol=crlf
-
 # Delphi source files
 *.pas text eol=crlf
 *.dfm text eol=crlf
@@ -13,12 +12,10 @@
 *.dcu binary
 *.bpl binary
 *.dcp binary
-
 # Documentation
 *.txt text eol=crlf
 *.md text eol=crlf
 *.UnitDoc text eol=crlf
-
 # Other text files
 *.rc text eol=crlf
 *.ini text eol=crlf
@@ -28,11 +25,9 @@
 *.xslt text eol=crlf
 *.css text eol=crlf
 *.js text eol=crlf
-
 # Model files
 *.mdl text eol=crlf
 *.bld text eol=crlf
-
 # Binary files
 *.exe binary
 *.dll binary