Support easy disabling of Docsy styles for HTML elements

[chalin]

Docsy defaults are good most of the time, but now and again it would be convenient if they could be easily disabled. For example, consider the following excerpt from https://github.com/google/docsy/blob/main/assets/scss/_content.scss:

.td-content {
    // ...
    table {
        @extend .table-striped;
        @extend .table-responsive;
        @extend .table;
    }
    // ...
}

To disable, we could use a special class named, say, td-initial to be used like this:

.td-content {
    // ...
    table:not(.td-initial) {
        @extend .table-striped;
        @extend .table-responsive;
        @extend .table;
    }
    // ...
}

In markdown, one could now write something like this to recover the base styles of one specific table:

| Element     | Attribute   |
| ----------- | ----------- |
| Header      | Title       |
| Paragraph   | Text        |
{.td-initial}

provided markdown attributes are enabled for blocks.

Thoughts @LisaFC et al.? I've come across the need for this and implemented it elsewhere -- see open-telemetry/opentelemetry.io#2359.

Tasks:

• Tables - [BSv5] Set tables to display:block and explain why #1477

Related:

• Docsy/User-guide "skin" styles should be factored out and/or dropped in some cases #1655
• Apply styling to all elements in td-content #1255 (comment)

[emckean]

+1 for this, it means (I think) that people will find it easier to keep in sync w/Docsy if they can just disable the styles they don't want rather than having to reapply patches?

[chalin]

Yes, that in particular.