website: Reworking of the "terraform plan" docs, and related pages

[apparentlymart]

It's been a long time since we gave this page an overhaul, and with our ongoing efforts to make plan and apply incorporate all of the side-effects that might need to be done against a configuration it seems like a good time for some restructuring in that vein.

The starting idea here is to formally split the many "terraform plan" options into a few different categories:

• Planning modes
• Planning options
• Other options

The planning modes and options are the subset that are also accepted by terraform apply when it's running in its default mode of generating a plan and then prompting for interactive approval of it. This then allows us to avoid duplicating all of that information on the terraform apply page, and thus allows us to spend more words discussing each of them.

This set of docs is intended as a fresh start into which we'll be able to more surgically add in the information about -refresh-only and -replace=... once we have those implemented. Consequently there are some parts of this which may seem a little overwrought for what it's currently describing; that's a result of my having prepared this by just deleting the -refresh-only and -replace=... content from our initial docs draft and submitted the result, in anticipation of re-adding the parts I've deleted here in the very near future in other commits.

This does include some additions that catch up with some changes in #28489 that weren't documented yet:

• terraform apply -destroy is now a thing, for symmetry with terraform plan -destroy and later on both with -refresh-only.
• Our -help output for various commands changed to match with the restructuring I made to the docs here, where terraform apply -help is more explicit that it's effectively running terraform plan internally and so to go look at the plan docs for information on modes and options.

[mildwonkey]

LGTM! I left a few minor suggestions, some of which are more subjective than others; I won't be offended if you opt to ignore any of those.

[apparentlymart]

Thanks for catching all of those, @mildwonkey! Unless I left a comment saying otherwise, I incorporated exactly what you proposed in the revised version that I'm about to merge.

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