Updated GaNDLF documentation for clarity

[Linardos]

Proposed Changes

• The changes are only on the documentation. Added an early explanation of config files and emphasized GaNDLF's purpose, plus added some edits further on for clarity and flow and to attract user's eye to the most important information.

Checklist

• CONTRIBUTING guide has been followed.
• PR is based on the current GaNDLF master .
• Non-breaking change (does not break existing functionality): provide as many details as possible for any breaking change.
• Function/class source code documentation added/updated (ensure typing is used to provide type hints, including and not limited to using Optional if a variable has a pre-defined value).
• Code has been blacked for style consistency and linting.
• If applicable, version information has been updated in GANDLF/version.py.
• If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file.
• Usage documentation has been updated, if appropriate.
• Tests added or modified to cover the changes; if coverage is reduced, please give explanation.
• If customized dependency installation is required (i.e., a separate pip install step is needed for PR to be functional), please ensure it is reflected in all the files that control the CI, namely: python-test.yml, and all docker files [1,2,3].

[github-actions]

MLCommons CLA bot All contributors have signed the MLCommons CLA ✍️ ✅

[sarthakpati]

Hey @Linardos, thank you for your contribution!

Please sign the CLA and we then can merge this PR.