Streamline Windows installer page

[timsneath]

Simplify manual installation instructions for Windows by:

• Creating a streamlined instructions page for installing Swift 6 on Windows without a package manager
• Relocating the laborious instructions for older (5.9 and earlier) Swift releases to a separate page
• Updating the top-level Windows installer page to include links to the new pages

I've also reviewed the instructions for modern Swift builds and:

• Harmonized them with the instructions for package managers (e.g. don't require CMake Project Tools)
• Changed the dependencies for Python and Git to point to the source locations, rather than the vended versions in Visual Studio (since Python in particular is no longer vended)

Fixes #845

[timsneath]

@swift-ci please test

[timsneath]

@compnerd Thanks for your feedback; please take another look and see if this addresses your comments. In particular, I've added a footnote to include your explanation on which Python version should be selected.

@swift-ci please test

[compnerd]

Perhaps use archived instead of old? But if that doesn't show up in the URL, I don't think that it really matters at all.

[timsneath]

@swift-ci please test again. thanks for all you do behind the scenes, my little unsung hero 🦸

[shahmishal]

@swift-ci please test

[timsneath]

@swift-ci please test