Fetching documentation data

[mageirakos]

Motivation

We should also fetch Rucio's documentation through github which will be used for context in answering general questions regarding Rucio.

Modification

1. a fetching script for most (most important) if not all the documentation (some might need to be compiled/exists in docstrings)
2. additional parsing might be needed to clean up the documentation (if so, create said parser as well)

These doc pages are then going to be indexed, so we might need to create the appropriate script for said job.
Related to #6 for the DocumentationSearchEngine

[mageirakos]

To not over complicate the issue, this will close once the code for fetching the documentation is complete. For the documentation parser (2) and any other code changes related to this we are going to open a new issue

[mageirakos]

closing based on latest pull request #8