Add GitHub workflows

[alexandratran]

Pull request checklist

Use the following list to make sure your PR fits the Besu documentation quality standard.

Before creating the pull request

Make sure that:

• All commits in this PR are signed off for the DCO.
• You've read the contribution guidelines.
• You've previewed your changes locally.

Describe the change

Replacing Circle CI workflows by Github Workflows.

Why? It's constantly blocking contributors who are not part of HL because they can't see what's failing and we have to fix for them as they can't re-run. Plus we regularly have issues of Circle not running despite the webhook firing well. Have this integrated with Github action provides all the features plus the ease of use and flexibility.

Issue fixed

Circle CI not running tests and non HL contributors not being able to see what's failing.

fixes #1146

Impacted parts

For content changes:

• Documentation content
• Documentation page organization

For tool changes:

• CircleCI workflow
• Build and QA tools (for example, lint or vale)
• MkDocs templates
• MkDocs configuration
• Python dependencies
• Node dependencies and JavaScript
• Read the Docs configuration
• GitHub integration

After creating your PR and tests have finished

Make sure that:

• You've fixed any issues raised by the tests.
• You've previewed your changes on Read the Docs
  and added a preview link.

Preview

https://hyperledger-besu--1147.org.readthedocs.build/en/1147/

[NicolasMassart]

@alexandratran everything passes except the links (we have to wait for splunk to revive their server but otherwise we can override)
LGTM Python fails... well because I removed all python stuff so it has nothing to build but it's not mandatory check and it will eventually detect that there's no python in the project once merged.
All the Circle CI required steps are showing as not running, because they were removed (replaced by a dummy one to prevent the Circle CI webhook to break until we merge). But I'm keeping them required for now until we merge this PR as otherwise it will impact other PRs.

[alexandratran]

The old architecture diagram was removed in #1158, so it doesn't need to be added back here. Otherwise, LGTM!