Fix Modal documentation about _handleUpdate method + move to public scope

* Change it's to its.

* Fix links

* Fix typos

updates the dom order in our docs to match (floated content comes first, but no need for that in flexbox)

- better table of contents and overall structure

- clearer examples of modal contents, longer modals, and modal options

* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

Since the "remote" option is no longer available in v4 the reference to the "loaded.bs.modal" event is no longer needed, too.

The modal backdrop positioning bug related to iOS' virtual keyboard doesn't seem to reproduce in iOS 8.0+.
(Possibly as a side-effect of https://bugs.webkit.org/show_bug.cgi?id=153224 )
Refs #9023
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Fixup for #19105
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- Switch from width to max-width for all widths to avoid scaling outside viewport bounds
- Rejigger the media queries for a more logical breakpoint for the large and small modal sizes
- Avoids changing the width of the default modal (nullifying #17794 and fixing #17581)

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Since `aria-label` is used, it's not necessary anymore.

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Mostly links.
Fixes #17056.

One extra `</p>` was out of place, plus couple of issues with `'s which caused some odd formatting.

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color

- Replaces manual use of .bd-callout with {% callout [type] %}
- Rearranged some callouts for proximity to others
- Turned long lists of callouts--like those on tooltips, plugings, etc--into a list because holy shit that's overwhelming