[ci] Update gitattributes and allow skipping docs generation for Windows CI workflows

[jimschubert]

This explicitly sets gitattributes so we should no longer get mixed CRLF and LF from some contributors on Windows, and on our GitHub workflow for master which verifies the integrity of the commit.

As a workaround for windows, we do have to skip some doc generation.

Many of our generators do things like:

    protected String projectFolder = "src/main";
…
    sourceFolder = projectFolder + File.separator +"groovy";

which would mean this location is output to documentation as src/main\groovy on Windows. There's not really a quick and painless resolution for this as it would require one of the following:

• Update all of these path partials to Path or File objects, or…
• Switch from getters on these relative path properties that return Strings to ones that return Path or File, and…
• Changing the contract of all generators to accommodate doc generation on Windows

Since the definition point does not need to know about OS-specific file handling, we could also remove our use of File.separator in all generators and stick to normal non-Windows separators. Even that is a ton of work, though.

Instead, this will add a --skip-docs option to ensure-up-to-date which will skip the regeneration of generator docs. Users on Windows who frequently see issues with docs can run this command instead;

./bin/utils/ensure-up-to-date --skip-docs

There's not really code changes, so I'll merge this to further evaluate the GitHub workflow. Just opening for community and core team awareness.

cc @OpenAPITools/generator-core-team

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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