Docs: Add quickstart user guide

[mlevesquedion]

This PR adds a user guide that shows how to configure and run the analyzer. The intent is to provide a "quickstart" tutorial that allows users to quickly grasp how they can start applying the analyzer.

• (N/A) [ ] Running against a large codebase such as Kubernetes does not error out. (See DEVELOPING.md for instructions on how to do that.)
• Appropriate changes to README are included in PR

[vinayakankugoyal]

Authentication

[mlevesquedion]

Are you referring to auth? I think I prefer auth here, because that is IMO the most specific label, being the name of the variable in the example. WDYT?

[vinayakankugoyal]

It says 'making the auth struct safe to log.' But there is no auth struct there is an Authentication struct.

[mlevesquedion]

In my understanding, the auth variable is a struct of type Authentication. I see your point, though. The sanitizer would work with values of type Authentication.

I've reworded it. WDYT?

[vinayakankugoyal]

I am thinking do we really need the line
For example, you may wish to define ...

[mlevesquedion]

I think it's good to make a nod to it here. Sources, Sinks and Sanitizers are (in my understanding) the 3 core concepts to understand for levee. Sources and Sinks are more fundamental, which is why they get most of the attention in this guide. But I think Sanitizers should at least get a mention.

[PurelyApplied]

Looks pretty good. I've got a handful of nits, but this is a good addition.

[mlevesquedion]

Thank you for your comments! PTAAL.

[mlevesquedion]

I've changed the config to use Value instead of Val when configuring the field tag, as per discussion here: #266 (comment)

I'm marking this PR a draft until that's merged. (Currently, using Value causes the configuration to be invalid, i.e. the tagged field won't be detected.)