Add a brief example

See #9612

Co-authored-by: NXavier Fernandez <xav.fernandez@gmail.com>

Signed-off-by: NSumana Harihareswara <sh@changeset.nyc>

Partially addresses #9194.
Signed-off-by: NSumana Harihareswara <sh@changeset.nyc>

Co-authored-by: NSumana Harihareswara <sh@changeset.nyc>

Furo provides a fully-fleshed-out right sidebar that provides an in-page
table of contents, that's scrollable and does not interrupt content.

Related to #7744 .

Co-authored-by: NPaul Moore <p.f.moore@gmail.com>

Related to #8307, #8115.

Related to #8792, #8076, #9020.

Related to #9020.

Per #9019.

In pip 20.3, the new resolver will be the default.
Therefore, this commit updates user docs to change
instructions for testing, migration, etc.

Towards #8937.

Co-authored-by: NPradyun Gedam <3275593+pradyunsg@users.noreply.github.com>
Co-authored-by: NSumana Harihareswara <sh@changeset.nyc>

updating user guide documentation with explanation and solutions to pip 
backtracking

docs: Add how to default to new resolver

Reformat a few spots in user guide

Fixes #8661.