Consolidate documentation under docs/ directory (#6310)

* Consolidate documentation under doc/ directory

Move user-facing guides from release_docs/ and doxygen/ into a single
doc/ root. release_docs/ now holds only release artifacts (changelogs,
history, release process, maintainer info).

- git mv release_docs/INSTALL*.md, USING_*.md, README_HPC.md,
  BuildSystemNotes.md, AutotoolsToCMakeOptions.md,
  HDF5_Library_2.0.0_Migration_Guide.md → doc/
- git mv doxygen/ → doc/doxygen/
- Update CMakeLists.txt: HDF5_DOXYGEN_DIR and add_subdirectory path
- Update CMakeInstallation.cmake: all install paths for moved files
- Update bin/make_vers: hardcoded doxygen/ path substitution
- Update doc/doxygen/CMakeLists.txt: EXAMPLES_DIRECTORY and comments
- Update README.md, CONTRIBUTING.md, SECURITY.md, config/README.md,
  release_docs/RELEASE_PROCESS.md: links to moved files
- Update doxygen .dox files: release_docs/ URLs for moved guides
- Rewrite release_docs/README.md for narrowed scope

* Add HDF5_DOCS_DIR variable for doc/ root path

Introduce HDF5_DOCS_DIR = \${HDF5_SOURCE_DIR}/doc so that
CMakeInstallation.cmake and future callers reference the doc/
directory symbolically rather than by hardcoded path.
HDF5_DOXYGEN_DIR is now derived from HDF5_DOCS_DIR.

docs/INSTALL_Windows.md

@@ -0,0 +1,14 @@
+# HDF5 Build and Install Instructions for Windows
+
+We recommend that users build, test and install HDF5 using CMake.
+
+Instructions for building and testing HDF5 using CMake can be found in the
+[INSTALL_CMake.md](./INSTALL_CMake.md) file found in this folder.
+
+For instructions on building and testing an application with HDF5, see
+[USING_HDF5_CMake.md](./USING_HDF5_CMake.md) file found in this folder.
+
+Users who want to build and run an application with HDF5 in Visual Studio
+without using CMake should consult the [USING_HDF5_VS.md](./USING_HDF5_VS.md) file. Building
+applications with the dynamic/shared hdf5 libraries requires that the
+`H5_BUILT_AS_DYNAMIC_LIB` compile definition be used.