Add Spring-specific styling to the Gradle Plugin's documentation

[wilkinsona]

The Gradle Plugin's documentation is using the default Asciidoctor styling. We should update it to use the Spring-specific styling that the reference documentation is now using.

[hsellik]

I'll look into this.

[hsellik]

@wilkinsona I have some questions:

1. It seems that the switch--item, switch--item selected and switch CSS classes need a change in spring-doc-resources. I couldn't find the spring.css file in the spring-doc-resources github page. Where is the proper place I can create an issue for this?

2. Documentation is also generated to pdf right now for Spring Gradle Plugin Documentation. Is that needed? (It's not there for reference documentation)

3. Should I wait with the PR for this until CSS classes get fixed and released?

For future reference:
I suggest replacing #7a2519 with #6BB344 in following CSS classes:

.switch--item.selected {
    background-color: #6BB344;
    color: #ffffff;
}

and

.switch--item {
    padding: 10px;
    background-color: #ffffff;
    color: #6BB344;
    display: inline-block;
    cursor: pointer;
}

and

.switch {
    border-width: 1px 1px 0 1px;
    border-style: solid;
    border-color: #6BB344;
    display: inline-block;
}

-->

[wilkinsona]

@DontSeeSharp Thanks for your work on this.

1. We can avoid changing that file. See 3.
2. Let’s keep the PDF, unchanged, please.
3. We can use the same approach as I used in the dependency management plugin’s documentation by adding CSS like this to this project.

[mbhave]

Closing in favor of PR #16327.