* Follow-up to #29095

This PR fixes the responsive containers that were added in #29095, originally stubbed out in #25631. Apologies to @browner12 for getting that wrong.

Fixes #25631.

* update navbar as well because we cannot reset all containers uniformly

* Update navbars example to include container-xl example to ensure containers match

* rewrite responsive containers docs, add table of max-widths

* Update container docs
- Move table up to the intro
- Remove the container example because it's actually hella confusing
- Update and link to grid example as a demo instead

Fix Readonly plain text form alignment

Improvement to better understand how utilities class name are generated by the (_api.scss) base in the line of code below:
`$property-class: if($property-class, $property-class, nth($properties, 1));`

https://github.com/twbs/bootstrap/blob/fc02932946424e986a72bb7b47044eab815851cb/scss/mixins/_utilities.scss#L21

From #28948.

"Popovers whose both title..."

Fixed minor grammatical error:

"... the best method get scroll ..."

=>

"... the best method to get scroll ..."

* Remove about generated button of file form
* Remove and merge layout examples of custom forms
* Clarify id/for
* Change some spacing classes for consistency

- Remove unsupported browsers fixes
- More consistent comment blocks
- Reorganise order of some blocks
- Add height fix for date/month/week/time inputs (#18842)
- Remove redundant type selectors
- Additional resets and theming options for `legend`
- Add date & color inputs to reboot page
- Use the same borders for text search, text inputs, textareas and date inputs
- Move code styling to reboot

Drop `color()`, `theme-color()` & `gray()` functions  in favor of variables. The functions just called a `map-get()` of a map where just the variables were defined. 

Also the `theme-color-level()` now accepts any color you want instead of only `$theme-colors` colors. The first value now is a variable instead of the `$theme-colors` key.