Convert MediaWiki content into raw text documents

[renoirb]

In order to allow the docs page to use git as edition system, we need to export the full content into raw text files.

High level description

• Export all content into simple as possible markdown
• Strip off any special feature, but keep raw content

Estimated tasks

• Find a way to grab content from MediaWiki and be able to work with content to format how we want to export it
• Set in place filter to strip Extension output that we won't need; e.g. splist, ...(?)
• Crawler that lists all pages
• Raw file dumper with path matching the url scheme, default is english (e.g. /css/properties/border-radius could write file in path css/properties/border-radius/en.md)
• If url ends by two letter code (e.g. zh, fr), make the file name to be called the slash before, inside the appropriate folder, e.g. /html/ja into html/ja.md
• Nice to have Find way to get all contributors from API, but don't make it part of import just yet.

[renoirb]

Could find a way to leverage MediaWiki Parsoid and add a new output manager from HTML to Markdown.

[renoirb]

Duplicate of #177 closing this one.

[renoirb]

• Migration scripts and library in webplatform/content-converter, and import script in webplatform/mediawiki-conversion
• Content migrated in webplatform/docs
• WPD namespace in webplatform/docs-wpd
• Meta namespace in webplatform/docs-meta