…tion (#32399)

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

* CONTRIBUTING: mention dist files

* Update CONTRIBUTING.md

Co-authored-by: Mark Otto <markd.otto@gmail.com>

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

Fix popover docs example

While technically it still works, we're already saying authors shouldn't do it later on, and we removed any examples of this. This was a straggler.

Co-authored-by: XhmikosR <xhmikosr@gmail.com>

Bump copyright year to 2021

Clarify the `$enable-shadows` option in our docs

Docs: Tweak the wording for collapse to indicate button is preferred/more semantic

Add two new variables for pagination border-radius values

* @babel/cli                   ^7.12.8  →  ^7.12.10
* @babel/core                  ^7.12.8  →  ^7.12.10
* @babel/preset-env            ^7.12.7  →  ^7.12.11
* @rollup/plugin-babel          ^5.2.1  →    ^5.2.2
* @rollup/plugin-commonjs      ^16.0.0  →   ^17.0.0
* @rollup/plugin-node-resolve  ^10.0.0  →   ^11.0.1
* autoprefixer                 ^10.0.2  →   ^10.2.1
* cross-env                     ^7.0.2  →    ^7.0.3
* eslint                       ^7.14.0  →   ^7.17.0
* eslint-config-xo             ^0.33.1  →   ^0.34.0
* eslint-plugin-unicorn        ^23.0.0  →   ^25.0.1
* find-unused-sass-variables    ^3.0.0  →    ^3.1.0
* hugo-bin                     ^0.66.2  →   ^0.68.0
* linkinator                    ^2.2.2  →   ^2.11.2
* nodemon                       ^2.0.6  →    ^2.0.7
* postcss                      ^8.1.10  →    ^8.2.3
* postcss-cli                   ^8.3.0  →    ^8.3.1
* qunit                        ^2.12.0  →   ^2.13.0
* rollup                       ^2.33.3  →   ^2.36.1