Docs: Added labels content

[oddlittlebird]

What this PR does / why we need it:

Took content from Ed's blog post and added it to the documentation. One topic is in Getting started, not sure that is the best place but did not see a better one. The other topic is in a new folder called Best Practices, which I hope you guys will add to over time.

It is needed because there is currently almost next to no content on labeling.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer: You might want to change some of the tone or remove some text to make it fit with the rest of your documentation.

Checklist

• [x ] Documentation added

[slim-bean]

This is great thanks @oddlittlebird!

I think we should also add links to these new pages via the table of contents:

For getting started:

https://github.com/grafana/loki/blob/master/docs/README.md and https://github.com/grafana/loki/blob/master/docs/getting-started/README.md

For best practices:

I think we should add a new entry in: https://github.com/grafana/loki/blob/master/docs/README.md and I think a new README.md in the new best-practices folder.

[codecov-io]

Codecov Report

Merging #1999 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1999      +/-   ##
==========================================
+ Coverage   64.46%   64.48%   +0.01%     
==========================================
  Files         131      131              
  Lines       10038    10038              
==========================================
+ Hits         6471     6473       +2     
+ Misses       3084     3083       -1     
+ Partials      483      482       -1     

Impacted Files	Coverage Δ
pkg/promtail/targets/tailer.go	78.40% <0.00%> (+2.27%)	⬆️

[owen-d]

Excited to get this in. @oddlittlebird Would you be willing to make @slim-bean's suggestions?

[oddlittlebird]

@owen-d , the links have already been applied.

[owen-d]

I missed Ed including his changes, we should have merged this a few days back.