[REQ] Merge OAS vendor extensions support pages

Is your feature request related to a problem? Please describe.

Migrate to single list of supported OAS vendor extensions.

Today, it seems like there are two different lists, where neither is a superset of the other.

Wiki: https://github.com/OpenAPITools/openapi-generator/wiki/Vendor-Extensions

This page includes x-generate-alias-as-model which is supported by all generators but not included in the following repo page.

Repo: https://github.com/OpenAPITools/openapi-generator/blob/master/docs/templating.md#extensions

This page includes x-enum-varnames and x-enum-descriptions which are supported by all generators but not listed in the wiki page.

Describe the solution you'd like

Single location which describes supported vendor extensions. It seems like it would be preferable to maintain this in a version controlled page. It would also be nice if this was a stand-alone page for vendor extensions, vs. being embedded in a larger templating page.

The page not chosen as the single location could be updated with a deprecation message and link to updated single list.

Describe alternatives you've considered

None.

Additional context

For reference, Redocly documents their OAS vendor extensions as follows:

• Repo: https://github.com/Redocly/redoc/blob/main/docs/redoc-vendor-extensions.md
• HTML Guide: https://redocly.com/docs/api-reference-docs/spec-extensions/

Compare the vendor-extension lists in the Wiki page and docs/templating.md#extensions, including the referenced Redocly examples. Choose the version-controlled location for a standalone combined list, then update the other page with a deprecation notice and link. Done means the supported extensions are documented in one location without losing any entries.