fix: enhance installation instructions and improve backward compatibility

[tomondre]

This pull request introduces updates to the README.md file to improve installation instructions and modifies the reana_jupyterlab/__init__.py file to address a compatibility issue. The most important changes include clarifying installation commands, enabling the server extension correctly, and fixing a validation error by exposing a server extension loading function.

Documentation updates:

• README.md: Updated installation commands to use python -m pip consistently for better compatibility
• Clarified the process for installing the extension in editable mode with development dependencies
• Adjusted the command for enabling the server extension to specify the correct module path (reana_jupyterlab.server).

Compatibility fix:

• reana_jupyterlab/__init__.py: Imported and exposed the _load_jupyter_server_extension function from the server module to ensure backward compatibility and resolve the following validation error:

(.venv) tomondre@tondrejk-1 reana-jupyterlab-extension % python -m jupyter server extension enable --py reana_jupyterlab.server
A `_jupyter_server_extension_points` function was not found in nbclassic. Instead, a `_jupyter_server_extension_paths` function was found and will be used for now. This function name will be deprecated in future releases of Jupyter Server.
Enabling: reana_jupyterlab.server
- Writing config: /Users/tomondre/Projects/CERN/reana-jupyterlab-extension/.venv/etc/jupyter
    - Validating reana_jupyterlab.server...
      X Validation failed: validation failed

[garciagenrique]

Why should we remove the python -m prefix here ?
And add it on lines 62 and 68 ? (python -m jupyter ...

This command installs the extension within the "current" python environment, doesn't it ?

[garciagenrique]

I would have run these commands in reverse order; first install all the requirements (including the dev ones), then enable dev mode (-e) to "link" any change to the current extension installation.

Am I missing anything ?