docs(mutations): Documentation for mutations

[joaogarin]

Document more detailed information on Mutations. Feedback welcome, this is all based on my experience going through tests and other places where i could find some information so highly appreciate feedback on things that can be wrong or not totally accurate.

Things to cover in terms of basic mutations

• CRUD Operations for Entities (CreateEntityBase)
• Mutations not related to entities (MutationPluginBase)
• Generating proper errors and violations
• Folder structure for the plugin system
• use statements and namespacing to your own module

[codecov]

Codecov Report

Merging #718 into 8.x-3.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             8.x-3.x     #718      +/-   ##
=============================================
- Coverage       79.8%   79.74%   -0.06%     
- Complexity      1269     1270       +1     
=============================================
  Files            192      192              
  Lines           3144     3145       +1     
=============================================
- Hits            2509     2508       -1     
- Misses           635      637       +2

Impacted Files	Coverage Δ	Complexity Δ
src/Plugin/GraphQL/Schemas/SchemaPluginBase.php	82.45% <0%> (-0.59%)	61% <0%> (ø)
src/GraphQL/Execution/QueryProcessor.php	89.47% <0%> (-0.53%)	70% <0%> (+1%)

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[fubhy]

I marked a few typos and grammaer issues as well as general style issues (back ticks for code references, variable names, etc.). Can you proofread the PR once again and fix the things I brought up?

Thanks, reaaally good job! I was a bit shocked about how much documentation you are adding here. Really cool!

[joaogarin]

Passed it all through a spell checker, also addressed the topics mentioned here. thanks for the feedback! ; )