chore: improve the KeymanWeb integration documentation to modernize the examples

[mcdurdin]

1. Get rid of the kmw variable noise.
2. Use example of promises
3. Consider adding integrity refs for the scripts
4. Remove the upgrade note now (perhaps move it to documentation?)

https://keyman.com/developer/keymanweb/

[ermshiperete]

See also keymanapp/help.keyman.com#365