Convert several .txt in release_docs folder to Markdown

[ajelenak]

Closes #6248.

Several .txt files in the release_docs folder with installation instructions are converted to Markdown. Cross-referencing in them was updated for the new .md files. A table of contents with section links was added to each one.

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Important

Convert several installation instruction files in release_docs from .txt to .md, updating cross-references and adding a table of contents.

• Conversion to Markdown:
  • Convert INSTALL, INSTALL_CMake, INSTALL_Cygwin, INSTALL_S3, INSTALL_Windows, USING_CMake_Examples, and USING_HDF5_CMake from .txt to .md.
• Cross-referencing:
  • Update cross-references in the converted files to point to new .md files.
• Table of Contents:
  • Add a table of contents with section links to each converted file.

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[ajelenak]

@brtnfld Need your approval again unfortunately.

[jhendersonHDF]

Should USING_HDF5_VS.txt be converted as well?

[jhendersonHDF]

It seems like a significant amount of useful information was removed from this file, especially in sections 5, 6, 7, 10, 11 and 13. I'm a bit hesitant about removing all of this information without it being somewhere else.

[ajelenak]

You have awaken a beast...😉 I found some .txt holdovers in the previously converted Markdown files. Hopefully all those are now updated.