Installation documentation improvements

### Suggestions

- [x] The "getting started" docs (or a similar suitable location) should point to the installation files in the release directory.
- [ ] The INSTALL* files need to be improved in both its breath of details, and the getting started needs to be made much more beginner accessible.
- [x] Convert txt files to Markdown
- [x] Merge unique Windows-specific notes from INSTALL_Windows.txt into the main installation guide.

Refactor INSTALL_CMake.txt:
- [x] Deprecate Section III: Move "Script Mode" (ctest -S) to a separate "Maintainer/CI Guide" or an appendix. It confuses standard users.
- [x] Promote Command Mode: Make the standard cmake -S . -B build command the primary "Advanced/Manual" method.
- [x] Extract Reference Tables: Move the massive "CMake Option Defaults" (Section VII) to a separate file, make it collapsible, or place it at the bottom of the document as an Appendix to improve flow.
- [x] Extract Testing: Move Section XIII (Regex Options) and XIV (Java FFM Testing) to a TESTING.md file.

Modernize Terminology:
- [x] Remove "Visual Studio Express."
- [x] Replace myhdfstuff with ${SOURCE_DIR} and ${BUILD_DIR} conventions.
- [ ] Standardize version placeholders to .
- [x] Clarify Dependency Management: Explicitly explain how to use HDF5_ALLOW_EXTERNAL_SUPPORT vs. pointing to local libs in the Manual Build section, as this is the most common failure point for users.