Emit upstream Installation instructions

[guineveresaenger]

This pull request is the scaffold implementation of generating installation docs for bridged providers from upstream source.

In this change, a new "language" called registry-docs is added to the bridge. (Note that "language" in this case is a bit of a misnomer - it is the tfgen CLI's input for determining which generating tactic to use - this is why schema is a language as well).

This change reads the upstream index.md file and emits it as a Pulumi package installation instruction file to a top-level folder called docs/ (the current registry standard). We also emit a minimal _index.md file. When these files exist in a provider, the registry will use those instead of the hard coded files existing in the registry currently.

This pull request includes a fair bit of TODOs in the docs parser - currently only the Hugo front matter is implemented, to show the scaffold. Transformations for this document to be Pulumi-friendly will follow in a separate PR.

This change does not affect any current use of the bridge - to flip it on, we'd add another Make target to a provider's Makefile, e.g. ./bin/pulumi-tfgen-openstack registry-docs --out docs/. I'd like us to consider adding this change despite the TODOs, so we can keep the changes somewhat atomic and reduce merge conflicts.

Closes #2082.

[codecov]

Codecov Report

Attention: Patch coverage is 19.23077% with 42 lines in your changes missing coverage. Please review.

Project coverage is 61.19%. Comparing base (ba48631) to head (0e694b9).
Report is 8 commits behind head on master.

Files	Patch %	Lines
pkg/tfgen/generate.go	17.85%	22 Missing and 1 partial ⚠️
pkg/tfgen/source.go	17.39%	17 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    pulumi/pulumi#2068      +/-   ##
==========================================
- Coverage   61.57%   61.19%   -0.39%     
==========================================
  Files         334      340       +6     
  Lines       44955    45215     +260     
==========================================
- Hits        27681    27668      -13     
- Misses      15750    16026     +276     
+ Partials     1524     1521       -3     

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