* docs: update plan command documentation. Fixes#19235
* docs: added a missing reserved variable name. Fixes#19159.
* website: add note that resource names cannot start with a number
* website: add some notes to the 0.12 upgrade guide
The announcement post contains the information about the temporary
situation where not all of the providers are compatible yet. Linking there
rather than duplicating the information in the upgrade guide means we'll
be able to update in one place as the situation changes.
The upgrade guide had its last major upgrade while we were preparing for
the alpha releases. Now that the upgrade tool is more complete we can
describe the required changes in terms of that tool, and also add
additional information about provider upgrades.
We will revise this at least one more time before v0.12.0 final, but this
is an interim copy of the upgrade guide intended to help those who are
testing the beta releases.
Based on some feedback on the initial draft, make some minor updates to
how information is presented in the preview upgrade guide for v0.12.
This also includes some minor copy-editing work to try to make the "voice"
more consistent between different sections of the guide.
