doc: add download page

[erisu]

Motivation and Context

Add the "Download" page, which is an ASF requirement.

Description

• Added Download Page
• Added a "releases.yml` file to easily update versions when new releases are made.
• Added jekyll-gfm-admonitions to support [!NOTE], [!TIP] etc.

Note

When a new release is made, Cordova typically publishes an announcement blog post. At the same time, the releases.yml file should be updated with the new release version.

Other important notes,

• Currently type: source is commented out for all packages. They will be uncommented after each new release following the new release process.
• All packages has defined customName. The customName field is expected to be removed after new release following the new release process. Previously all packages were following the npm's package naming convention while future packages will follow a new naming convention. <repo-name>-<type>-<version>.<extension>.

Testing

npm run serv

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary