[1.1.0] Support Navigation module

Problem/Motivation

When using UI Suite DSFR with the core Navigation module, there are side effects.

- links get an underline:

[href] {
    background-image: var(--underline-img), var(--underline-img);
...
}

- the header has a higher z-index:

@media (min-width: 62em) {
    .fr-header {
...
        z-index: calc(var(--ground) + 750);
    }
}

Steps to reproduce

Proposed resolution

- Ensure DSFR CSS does not impact Navigation
- Override DSFR z-index or override Navigation z-index.

Remaining tasks

User interface changes