(5/5) The site: Android, Windows and Linux in development

[ErickXavier]

Description:

The site's part of bringing Sill to more systems. It is independent of the four other pull requests (`feat/android-client`, `feat/windows-client`, `feat/linux-host`, `feat/windows-host`) and can merge before or after them.

**It offers nothing that isn't released.** Every addition says the new systems are in development, with no download, and sits between `<!-- Other systems -->` markers. docs/release-checklist.md says what a republish and a release do with them.

- **index.html:** an "Android, Windows and Linux" section, covering the Android and Windows clients and a Windows or Linux PC as the host.
- **download.html:** a card saying the same, and that this page's Sill.dmg is always Sill for Mac.
- **support.html:**
  - where to report a problem with a build from source;
  - the renamed items under both names: the Mac's "Pair a Device… (Pair iPhone or iPad… in Sill for Mac 0.5.1 and earlier)" and "A Device Needs Sill Updated", and the device's "Add a Host… (Add a Mac… in Sill 0.5.1 for iPhone and iPad and earlier)".
- **privacy.html:**
  - the policy will cover the new systems when they are released;
  - what each keeps on its device, as built now: Android's key sealed by the Keystore, the Windows client's file in AppData, the hosts' settings file;
  - Android asks for no permission but network access.
- **terms.html:**
  - builds of the new systems are Apache-2.0;
  - a host shared with FFmpeg's GPL build is GPLv3, and the Windows client's FFmpeg is LGPL;
  - the Android, Windows and Linux trademark lines.