feat: Change license from MIT to Apache-2.0

- Added LICENSE.md file with Apache License 2.0 text.
- Created NOTICE file for attribution of third-party components.
- Updated README.md to reflect the new license and included details about the Apache-2.0 license.
- Documented the license change in the diary entry for July 12, 2026.
- Updated installation instructions in getting-started.md to reflect per-user installation.
- Added reusable license header to all source files.
- Updated various documentation files to mention the new Apache-2.0 license.
- Changed legal mentions in example content to reflect the new license.

NOTICE

@@ -0,0 +1,19 @@
+GitCover.WebStaticBuilder
+Copyright 2026 GitCover Commons gUG (haftungsbeschränkt)
+
+This product includes software developed by GitCover Commons gUG (haftungsbeschränkt)
+(https://gitcover.org).
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not
+use this product except in compliance with the License. A copy of the
+License is provided in the file LICENSE.
+
+Third-party components distributed with this product:
+
+  - Markdig (BSD-2-Clause)
+    https://github.com/xoofx/markdig
+  - YamlDotNet (MIT)
+    https://github.com/aaubry/YamlDotNet
+
+The above components retain their respective licenses; their license texts
+and notices are reproduced in the documentation and source distributions.