Documentation improvements

[nariman87]

Context for changes

Major readthedocs improvements as listed below and some minor README.md updates.

• API details and inheritance diagrams should be now correctly displayed:
  However, inheritance diagrams are still limited to what was instructed for classes in original Python files. See for example fp. codes the only package where two top classes were specified. See here and here for example on how to set up your classes for the best inheritance diagram experience.
• "Edit on Github" links were fixed
• The general style and section structures were made more consistent with the company requirements and other packages such as StrawberryFields.
• Fixed the documentation links in README.md
• All C++ imports are now placed within a try block to avoid interrupting non-compiled installations, such as the one currently used by readthedocs.
• Updated doc/conf.py, doc/dev_requirements.txt, and doc/Makefile, among other minor file changes, to achieve the above.
• Relative path cannot be used in README.md logos:
  Was replaced with Github-hosted links.
• Code coverage and quality scores were improved to arguably acceptable levels.

Example usage and tests

• Not Applicable

Performance results justifying changes

• Not Applicable

Workflow actions and tests

• Not Applicable

Expected benefits and drawbacks

Expected benefits:

• More consistent and modern-looking documentation when compared to other Xanadu and open-source products.
• The codefactor quality score is now increased to B-.

Possible drawbacks:

• For now, we are ignoring C++ APIs.

Related Github issues

• Not Applicable

Checklist and integration statements

• My Python and C++ codes follow the coding and commenting styles of this project as indicated by existing files. Specifically, the changes conform to given black, docformatter and pylint configurations.
• I have performed a self-review of these changes. I have checked my code and corrected misspellings to the best of my capacity. I also confirm that I have already merged other branches into this branch as required.
• I have added context for corresponding changes in documentation and README.md as needed.
• I have added new workflow CI tests for corresponding changes and these pass locally for me.
• I have updated _version.py based on semantic versioning. I recognize that the developers may create a Special Release including my changes. I suggest a Special Release PR is essential immediately following these changes.