[feat][docs][website] Initial docusaurus based site

PR checklist

• 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\.
• Filed the PR against the correct branch: master, 3.4.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

To evaluate:

cd website
yarn start

This will automatically open http://localhost:3000 with the generated site.

This PR include some changes to the docs under ./docs to support the docusaurus configuration. Every generator markdown under ./docs/generators includes the docusaurus metadata header, although the tool doesn't currently support indexing markdown documents in subdirectories under ./docs (as they support localization and walking the tree becomes difficult).

There are some todo items, either before or after this is evaluated:

• Include bin/utils/export_generators_docusaurus_index.sh in CI
• Update CI to deploy to GitHub Pages
• Add attribution info for Font-awesome to footer (svgs pulled from encharm/Font-Awesome-SVG-PNG
• Move users list to editable yaml/page in same way as team (team.yml)
• Create an introduction guide
• Create more tutorials for extending the generator
• Better document openapi-generator-online
• Feature matrix see #503
• List OpenAPI Specification features we support see #503
• Script out the copy of CODE_OF_CONDUCT.md and CONTRIBUTING.md from project root to docs/conduct.md and docs/contributing.md (where these include docusaurus headers.
• Migrate Wiki FAQ
• Migrate (or rewrite) Wiki How to add a New Generator to Extending section
• Migrate (or incorporate into templating.md) Wiki Mustache Variables
• Migrate Wiki Vendor Extension

Questions:

Can the following be left on the Wiki?

• Wiki Server Stub docs Q: Can we move the few notes and rely on users to query CLI for generation options?
• Wiki Git Branches
• Wiki Release Checklist
• Migrate Wiki Samples Folder Q: What does this page provide to users?
• Migrate Wiki Pull Request Checklist to Contributing section. Q: Should this be removed as redundant with PR/Issue templates?
• Migrate Wiki Integration Tests to contributor section

TODO items (so I don't forget)

• Create issue to dump templating information (embeddedTemplateDir/templateDir) via config-help
• Create issue to dump supporting files used by generator (useful for template authors)
• Create issue to dump OpenAPI supported features per generator (included all three above items in single feature request: #1811)