Use current latest channel as stable channel instead

[eightfilms]

Closes #116

My observation of the needs of both internal and external devs seem to suggest that it might be better to always provide the latest binaries regardless of whether they passed tests.

It also makes sense to name the releases that passed integration tests as compatible and therefore stable, since they have basically passed all known success and failure tests. Until we have a better definition of what constitutes a stable toolchain, I propose we use this definition in the meantime.

The recent PR that defines the latest toolchain #95 will be used as the stable toolchain instead while the latest toolchain will always provide the latest binaries, regardless of whether they passed integration tests.

To accomplish this,

• I brought back parts of the old index-versions.sh that indexes latest versions indiscriminately as a separate index-versions-latest.yml workflow to publish the latest channel. This also runs every 30 minutes.
• I made necessary changes to the existing workflow(s) that publishes the latest channel, to publish the stable channel instead. This is done through changes to names of jobs, files, etc. within the YAML configuration files.
• Updated docs (CHANNELS.md). This should eventually be within an mdbook.

[eightfilms]

Closing this PR after a discussion with @mitchmindtree.

fuelup's mission should be to provide tested and reliable toolchains, so therefore we shouldn't need a separately latest toolchain that purely has the latest binaries since they might fail integration tests with each other. Any more cutting edge than latest should mean either building from source or using custom toolchains (soon ™️)

If we use the mission as guidance for fuelup's development, the changes this PR make would be a regression.