Add "docExtension" option to customize generated file extension for docs

Created by: rienafairefr

PR checklist

• [X ] Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [X ] Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

moved from https://github.com/swagger-api/swagger-codegen/pull/7681 there might still be rebase issues, but it looks fine to me

This enables to generate some supporting file (documentation) by specifying a different file extension for it, useful when you're using customised templates with e.g. Restructured Text in it, and don't want to have the files forced to be named README.md or Api.md