Add icon to static pages, configurable

[joachimnielandt]

Added a configurable icon to the static pages. The icon is not shown when not supplied. Added help text indicating it needs to be a font-awesome icon.

@fxprunayre not sure what happens for submenus - the context leads me to believe only top / footer items are allowed for now, and that seems to work fine on my end.

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
• 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

[fxprunayre]

@fxprunayre not sure what happens for submenus - the context leads me to believe only top / footer items are allowed for now, and that seems to work fine on my end.

Works fine in submenu too

eg.

[
  "gn-site-name-menu",
  "gn-portal-switcher",
  "gn-search-menu",
  "gn-map-menu",
  {
    "Quick search": [
      "dada"
    ]
  },
  "gn-contribute-menu",
  "gn-admin-menu"
]