Create all in one Windows Leo.exe automatically with GitHub Actions

[maphew]

gh-act branch is now successfully using GitHub Actions to

• automatically build a new everything-included Leo.exe on every push to that branch.
• copy the results somewhere so people can download and use it.
• set so that builds only happen on tagged commits
• merge to devel branch
• document for developers
• add user-facing instructions to Leo docs (and website)

https://help.github.com/en/actions/configuring-and-managing-workflows/persisting-workflow-data-using-artifacts

part of #229 .

[maphew]

Works! but getting to the archive in order to download is a bit of work. There isn't a stable url[0]

Getting to the latest leo.exe:

1. Navigate to Actions
2. Select Build standalone leo.exe from workflow list in the left table of contents
3. Look for a recent event with a checkmark and click on it
4. Look for Artifacts heading and under it: leo-windows-exe in blue.

[0]: Pending this upstream feature request actions/upload-artifact#21 (comment)

[maphew]

@edreamleo The user facing instructions are done, see comment before this one. I'm not sure about the best place to put it in the docs.

[edreamleo]

@maphew Put the new instructions wherever you like. I'll review the changes, if you like.

[maphew]

Ok, added to download.html in d9943f8.

[maphew]

Putting to sleep until I or someone else has time and initiative to finish it. What's here so far does work.