Fix the grid on the homepage of the documentation

[MichelGabriel]

Fix the grid on the homepage of the documentation.

Rename .grid to .gn-grid to avoid conflicting classnames, and some additional styling so content doesn't break between columns.

When the documentation is released and deployed online styles were conflicting. The styles are renamed but the conflict didn't occur locally, so hopefully it's solved.

Before

After

Checklist

• I have read the [contribution guidelines](https://github.com/geonetwork/core-geonetwork/blob/main/CONTRIBUTING.md
• Pull request provided for main branch, backports managed with label
• [x Good housekeeping of code, cleaning up comments, tests, and documentation
• Clean commit history broken into understandable chucks, avoiding big commits with hundreds of files, cautious of reformatting and whitespace changes
• Clean commit messages, longer verbose messages are encouraged
• API Changes are identified in commit messages
• Testing provided for features or enhancements using automatic tests)
• User documentation provided for new features or enhancements in mannual
• Build documentation provided for development instructions in README.md files
• Library management using pom.xml dependency management. Update build documentation with intended library use and library tutorials or documentation

[josegar74]

@MichelGabriel I see the issue in https://docs.geonetwork-opensource.org/4.4/, but locally is confusing:

• With mkdocs serve I don't get the issue even without the changes.
• With mike serve I get the issue even with the changes.

[MichelGabriel]

Thank you, then I will also test with mike serve and try to fix it

[jodygarnett]

Looks good to me, I appreciate the use of requirements.txt to force the use of appropriate mkdocs material version to accomplish the fix.

I will squash and merge to allow for easier backport (and remove noise as content was edited and then removed).

[jodygarnett]

@MichelGabriel I am using this to test backport.yml automation