Finish keyboard install page (aka universal link infrastructure) for macOS,Android,iOS

[mcdurdin]

Finish keyboard install page (aka universal link infrastructure) for:

• macOS (fix: fix: keyboard install page for macOS keyman.com#172)
• Android (fix: feat(android): Android install page keyman.com#185)
• iOS (fix: feat(ios): install page keyman.com#184)

Originally posted by @mcdurdin in #87 (comment)

[mcdurdin]

@darcywong00 can you advise on Android requirements for the Install page (e.g. https://keyman-staging.com/keyboards/install/khmer_angkor) when Keyman is not installed?

@jahorton can you advise on iOS requirements for the Install page (e.g. https://keyman-staging.com/keyboards/install/khmer_angkor) when Keyman is not installed?

[jahorton]

Well, for starts... that formatting.

The basic install link checks out from live devices. (Oddly, Safari thinks it's invalid every time I try to test it in the Simulator.)

Long-pressing the second link brings up the "Open in Keyman" command to install the package. Weird that it wasn't launching Keyman without it; maybe that bit of universal link stuff never got fully completed properly? At any rate, that should be all that would be needed; the 1 + 2 would cover it.