[cpp][qt] Start cleanup of cpp qt readme

[xconverge]

Unless I am severely confused... this readme needs some love, so just starting a little bit of the no brainer cleanup.

@etherealjoy @MartinDelille

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[MartinDelille]

Indeed.

After a quick look here are some suggestion:

• Add missing code block language (c++ and yaml)
• Fix code indentation

[xconverge]

Sounds good, I think this is a better sane starting point now :)

[MartinDelille]

The links in the following paragraph are dead:

• https://github.com/xconverge/openapi-generator/tree/updateCppQtReadme/samples/client/petstore/cpp-qt#documentation-for-api-endpoints
• https://github.com/xconverge/openapi-generator/tree/updateCppQtReadme/samples/client/petstore/cpp-qt#documentation-for-models

I would suggest to remove the link since there is specific generators for the documentation.

[xconverge]

@MartinDelille I think this section could be removed too?

"## Documentation for Authorization"

[wing328]

I think we should keep it to show users whether this API requires authorisation or not.

[wing328]

LGTM

[wing328]

cc @ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03) @muttleyxd (2019/08)