Improved instructions for getting artifact builds from try server

Ben and I had an exchange about having instructions that could be used by educated users for using try builds for testing patches - but the current instructions are incomplete. References:

• issue #99
• https://developer.thunderbird.net/thunderbird-development/building-thunderbird/artifact-builds
• https://developer.thunderbird.net/thunderbird-development/fixing-a-bug/try-server#get-an-installable-build-from-a-try-run
• https://bugzilla.mozilla.org/show_bug.cgi?id=399475#c29

Draft of instructions might look like:

When the build at https://treeherder.mozilla.org/jobs?repo=try-xxxxxxxxxx is complete (normally takes xx hours)

• Click the green "B" (for binary) next to one of the following: "Windows 2012 x64 opt", "Linux x64 opt", "OS X Cross Compiled shippable opt" (unless instructed to use a debug build)
• In the black header below click "Artifacts and Debugging"
• In the Artifacts section, to download the install file click on target.installer.exe (Windows), target.tar.bz2 (Linux), or target.dmg (Mac)
• Install the downloaded file

Read the existing artifact-builds and try-server instructions at the two developer.thunderbird.net links, then compare them with issue #99 and the Bugzilla reference. Update the relevant documentation with the draft Treeherder steps for Windows, Linux, and macOS downloads, and verify that the instructions cover downloading and installing a completed try build.