Backport of Update docs and add warning for -get-plugins into v0.14

[teamterraform]

Backport

This PR is auto-generated from #27169 to be assessed for backporting due to the inclusion of the label 0.14-backport.

The below text is copied from the body of the original PR.

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As of Terraform 0.13+, the get-plugins command has been superseded by new provider installation mechanisms, and general philosophy (providers are always installed, but the sources may be customized). Update the init command to give users a warning if they are setting this flag, to encourage them to remove it from their workflow, and update relevant docs and docstrings as well.

Fixes #27092

Looking to backport to 0.14 (which would give 0.14.1+ users a warning when using this flag), and remove the flag completely in 0.15 after this.

Also looking for:

• Are the reccs here the preferred recommendations for people using this flag?
• Approval/Nay-saying on the "add a warning, then remove completely" plan
• Should this be backported to 0.13 as well?

[codecov]

Codecov Report

Merging #27176 (304f96d) into v0.14 (a013a3a) will decrease coverage by 0.00%.
The diff coverage is 53.84%.

Impacted Files	Coverage Δ
command/init.go	62.33% <53.84%> (-0.39%)	⬇️
dag/marshal.go	53.42% <0.00%> (-1.37%)	⬇️

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