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Changes grid and container sizes to `px`, as the
viewport pixel size does not depend on the font size.

The actual em values were inconsistent with the docs,
while the docs were not the same as the comments:

* `sm` breakpoint was 34em (544px) not 480px.
* `lg` container max-width was 60rem (960px), less gutter than `md`.
  Changed to 940px, same as Bootstrap 3.
* `xl` container max-width was 72.25rem which is 1140px not 1156px.
  Changed to 1140px matching the comment but not the docs.

Addresses #17070 and #17388.

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Closes #17699

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resolves #17609

Closes #17214.

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This adjusts the load path for the ``configBridge.json`` file from one
that's relative to the user running the command (eg ``jekyll serve``),
to one that's relative to the ``source`` configuration setting for
Jekyll.

The result is that the user can now have a (customised) ``_config.yml``
for Jekyll anywhere on her filesystem and point to Bootstrap's
``docs`` directory to use as the ``source``.  Previously, in order to
customise it, the original ``_config.yml`` needed to be modified inside
(a forked) Bootstrap, and the ``jekyll`` command could only be run at
the root of the Bootstrap package as the original file path to
``configBridge.json`` was only valid there.

The existing behaviour is not affected.

Fixes #17196
Closes #17510

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Move them to footer, and remove duplicate ones.

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