* 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

- Add/replace callout for color and AT with include

This one was overlooked at edit

See #17719 and #19199

[ci skip]

[skip sauce]

/fyi @mdo

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As per our browser compat policy, we only claim support for latest
versions of Opera on desktop. Current Opera is version 34. From testing, IE11 does support `pointer-events:none` just fine now.

to harmonise it with the callout given in pagination

...as per the text immediately preceding this example

Fix #17234

[skip sauce]

- 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

/cc @cvrebert @patrickhlauke

- 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