Merge: doc/commands: introduce INI commands

This PR introduces commands related to `package.ini` files:
* `CmdIniDescription`: return the package description from the `package.ini` file
* `CmdIniGitUrl`: return the package Git URL from the `package.ini` file
* `CmdIniCloneCommand`: return the package `git clone` command from the `package.ini` file
* `CmdIniIssuesUrl`: return the package issues URL from the `package.ini` file
* `CmdIniMaintainer`: return the maintainer name from the `package.ini` file
* `CmdIniContributors`: return the contributors list from the `package.ini` file
* `CmdIniLicense`: return the license string from the `package.ini` file
* `CmdLicenseFile` and `CmdLicenseFileContent`: return the LICENSE.md file and its content
* `CmdContribFile` and `CmdContribFileContent`: return the CONTRIBUTING.md file and its content

Pull-Request: #2657
Reviewed-by: Jean Privat <jean@pryen.org>