Fix docs and update to FAKE 5

[kblohm]

Hi,
in #107 you told me that the docs were broken for some time now, so i thought i might fix that.
At least for me (on windows) they build just fine now.
While i was at it i also updated the build to use FAKE 5.
Some stuff that might still need some work:

• I did not test the release targets as they require git permissions.
• build.sh might need some more work, i am not very firm in bash-scripts :/.
• .travis.yml and appveyor.yml might not be quite right

[kblohm]

Any ideas why travis is not working?
I am probably doing something wrong in build.sh. But i have no way to test this locally except for git-bash and that is working fine :/. Some help would be appreciated :)

[panesofglass]

I'll try on my Ubuntu subsystem later today. Thanks for doing this!

[kblohm]

Soo.. that travis problem was apparently a bug in the dotnet tooling and how they try to resolve dotnet sdk tools that are installed to a none default folder. So i changed the way to bootstrap FAKE from the global tool to a build.proj file.
I also added a HostDocs target to host the docs with AspNetCore, so we dont need to generate the docs with a different root just for testing.

[kblohm]

Ok, its finally working :)

[panesofglass]

I merged and added a few clean up commits. I think I broke the HostDocs, but it fixes the paths for the published docs. I am not sure the best way to resolve using local or fixed paths.

[kblohm]

Mh, i was pretty sure gh-pages are fine with relative paths :/.

[panesofglass]

@kblohm it works okay with relative paths, but the paths in the template were relative to the root, and the root was not set to /FSharp.Control.Reactive, as it needs to be for github pages, i.e. https://fsprojects.github.io/FSharp.Control.Reactive/index.html. The old generate.fsx used an #if RELEASE for using the root, but I'm uncertain how it used that correctly.

[panesofglass]

In any case, you fixed the documentation generation, and it's now updated! Thanks!

[panesofglass]

Also, @kblohm, apologies for not cleanly merging, leading this to appear unmerged.