docs(backend): improved backend README

[dandawg]

Description of your changes:
This PR makes the following improvements to the backend/README.md file, which are broadly aimed at making the file more user friendly (easier to follow and clearer to understand):

1. Added an Overview section (intro) to make it more clear what the readme's purpose is.
2. Added a Prerequisites section to help people know up-front what they need. (In the future, it may be good to give even more details here, for example on versions). The go-licences pre-req is particularly important to building the docker image--without it users wait for the build to fail, then troubleshoot.
3. Organized content into major sections with more appropriate header levels.
4. The Code Style section was in the previously in the middle of the 'building and testing' commands--I moved it to it's own Contributing section at the end.
5. Fixed make all command to run from perspective the root (pipelines) directory (to be consistent with the other commands).
6. This also clarifies what is happening when some commands are run to help understanding / prevent misunderstandings.

Checklist:

• You have signed off your commits
• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[google-oss-prow]

Hi @dandawg. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign hbelmiro for approval. For more information see the Kubernetes Code Review Process.

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[hbelmiro]

/ok-to-test

[github-actions]

Approvals successfully granted for pending runs.

[hbelmiro]

Thank you @dandawg.
I left a few comments.

[dandawg]

Thank you @dandawg. I left a few comments.

Great suggestions. I added in the changes @hbelmiro

[hbelmiro]

/lgtm
/ok-to-test