[BuildStream] Re-structure of BuildStream Gitlab policy document

[Laurence Urhegyi <Laurence Urhegyi codethink co uk>]

Hi,

I'd like to invite you to review the following MR with some updates to the 
structure BuildStream's Gitlab policy document (not the actual policies 
themselves):

https://gitlab.com/BuildStream/nosoftware/alignment/merge_requests/7/commits

### Description

I've heard complaints recently that people did not know how to follow the 
policies of the project related to Gitlab. People found the policies to be 
difficult to consume, so this MR is an attempt to simplify them and make them 
more accessible.

### Proposed changes

Changes proposed in this merge request:

* Updates old HACKING links to CONTRIBUTING
* Cleans up all of the links to be neat hyperlinks in markdown format , instead 
of full links
* Massively reduces amount of text relating to templates and also opening / 
closing tickets
* Removes information about nosoftware labels

Note: I have left in there what I believe to be some outdated info on the 
severity labels and impact labels since Tristan changed them. However I'm not 
too sure how there are actually being used at the moment, so would appreciate 
some input there. The new world order for severity labels really does not make 
sense to me....perhaps we can consider changing it back...but that's a 
discussion for elsewhere....

Also, I think we may need to outline better how to flag a bug you are raising as 
'high priority' so that the development team take a look at it.

Admittedly, some of the finer details have been removed. But I believe it is 
worth it in order to have a more accessible and easy to digest policy.

Thanks,
Laurence