Backport contents from v2020pre

Backport stuff that is relevant for v2019:

• d5ce6c0b Document the cross build toolchain
• 353d4888 sysroots-and-devroots: Link to the cross toolchain doc
• b23e19d9 sysroots-and-devroots: Link to usage instructions without ade
• fbff6863 sysroots-and-devroots: Clarify that multiple [sys|dev]roots can coexists
• 8c867513 sysroots-and-devroots: Mention app-bundle agents as well
• b91e5676 sysroots-and-devroots: Add links to release artifacts
• 1d66e812 sysroots-and-devroots: Point out that amrhf/devroot is preinstalled on sdk
• 5078559c sysroots-and-devroots: Clarify remote debugging tradeoffs
• 1bb442a6 sysroots-and-devroots: Describe requirements for devroot in third-party SDKs
• c857d0ce Sysroot-and-devroots: Rephrase links
• 1ff1a156 Stakeholder feedback: docs/cross-build-toolchain.md
• 27ffe7a0 Add warning about persistent disk overwriting changes
• 3ea0b226 Add newline between two warnings
• fed6a729 README: Drop old stuff and point to Docker to build the documents
• 719ccfd7 vm-netboot-server: add initial setup
• d852e503 vm-netboot-server: insert into FAQ section
• db063151 hotdoc-html-2-pdf: Set up fonts to better handle unicode data
• b44bd3e5 hotdoc-html-2-pdf: Keep links clickable