feature: add style for mkdocstrings backlinks

[pawamoy]

Styles for zensical/zensical#964.

We can also consider adding these styles for tree-style backlinks:

/* Tree-like output for backlinks. */
.doc-backlink-list {
  --tree-clr: var(--md-default-fg-color);
  --tree-font-size: 1rem;
  --tree-item-height: 1;
  --tree-offset: 1rem;
  --tree-thickness: 1px;
  --tree-style: solid;
  display: grid;
  list-style: none !important;
}

.doc-backlink-list li > span:first-child {
  text-indent: .3rem;
}
.doc-backlink-list li {
  padding-inline-start: var(--tree-offset);
  border-left: var(--tree-thickness) var(--tree-style) var(--tree-clr);
  position: relative;
  margin-left: 0 !important;

  &:last-child {
    border-color: transparent;
  }
  &::before{
    content: '';
    position: absolute;
    top: calc(var(--tree-item-height) / 2 * -1 * var(--tree-font-size) + var(--tree-thickness));
    left: calc(var(--tree-thickness) * -1);
    width: calc(var(--tree-offset) + var(--tree-thickness) * 2);
    height: calc(var(--tree-item-height)  * var(--tree-font-size));
    border-left: var(--tree-thickness) var(--tree-style) var(--tree-clr);
    border-bottom: var(--tree-thickness) var(--tree-style) var(--tree-clr);
  }
  &::after{
    content: '';
    position: absolute;
    border-radius: 50%;
    background-color: var(--tree-clr);
    top: calc(var(--tree-item-height) / 2 * 1rem);
    left: var(--tree-offset) ;
    translate: calc(var(--tree-thickness) * -1) calc(var(--tree-thickness) * -1);
  }
}

That's what gives a style similar to the output of the tree command:

% tree -L2
.
├── CHANGELOG.md
├── CODE_OF_CONDUCT.md
├── config
│   ├── coverage.ini
│   ├── git-changelog.toml
│   ├── pytest.ini
│   ├── ruff.toml
│   ├── ty.toml
│   └── vscode
├── CONTRIBUTING.md
[...]

Otherwise backlinks are just rendered as bullet points.

This CSS wasn't part of autorefs/mkdocstrings previously, so not sure if we should make it the default.

[squidfunk]

Changes look good! Regarding tree-style backlinks I trust your judgment.