Generate docs for datasources

[roaks3]

Part of hashicorp/terraform-provider-google#25987

This adds docs generation for datasources that are generated. The structure is fairly standardized to begin with, and I tried to use the wording that was most commonly used thus far.

The main parts that were more ambiguous:

1. Examples are hard because (so far) we don't ask the contributor to provide any explicit datasource examples. So to generate the datasource config for tests, we assume we can pull values off of the resource, which works pretty well. I've chosen to do the same for docs simply because it avoids trying to do something more complicated with literals (for example, using a full set of sample values per type, or trying to parse values from the resource example config).
2. The field descriptions are using what we have for the resource, so some of them are worded slightly weird for a datasource, like referencing creation, and usually include more content than is necessary for the datasource. I considered using firstSentence, but I'm a bit worried about how reliably it will be, especially since most contributors will not check what documentation was generated.

I think in its current state, this should be fine to advance forward and help make datasource creation much easier, but open to feedback of course.

For awareness: this started out as a heavily AI-assisted solution, but by the end I was making a lot of hand edits. The result is a bit of a mix + manual verification and testing that I did.

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit b015113:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	7 files changed, 159 insertions(+), 106 deletions(-)
google-beta provider	View Diff	7 files changed, 159 insertions(+), 106 deletions(-)

Errors

google-beta provider:

• Error reading file website/docs/d/vmwareengine_datastore_html.markdown

[roaks3]

I think the template check is actually a bug in the check:

newtmplfiles=$(git diff --name-only --diff-filter=A origin/${GITHUB_BASE_REF} HEAD -- ../../mmv1 | grep .tmpl | sed 's=^=../../=g')
  if [ ! -z "$newtmplfiles" ]; then
...

Looks like it only considers new templates, but would otherwise flag any of our other generic templates.