Improve transpiler documentation (#9087) (#9469)

* Improve transpiler documentation

This commit expands the module documentation for the qiskit.transpiler
module to explain details of working with preset pass managers and how
to modify stages for custom workflows. It also updates some of the
details about the stages which were a bit stale on the exact behavior of
the transpiler.

* Apply suggestions from code review

Co-authored-by: Jake Lishman <jake@binhbar.com>

* More refactors from code review

* Update reference label for stages section

* Add section headers to scheduling stage docs

* Fix lint

* Fix typo

* Update examples to show stochastic passes

* Fix rebase typos and example issues

* Update qiskit/transpiler/__init__.py

* Apply suggestions from code review

Co-authored-by: Kevin Krsulich <kevin@krsulich.net>

* Fix trailing whitespace

* Expand init stage definition

* Apply suggestions from code review

Co-authored-by: Kevin Krsulich <kevin@krsulich.net>

* Simplify target section intro

* Highlight legacy views are for backwards compatibility

* Updating layout and routing for extra details

* Apply suggestions from code review

Co-authored-by: Kevin Hartman <kevin@hart.mn>

* Update qiskit/transpiler/preset_passmanagers/__init__.py

Co-authored-by: Kevin Hartman <kevin@hart.mn>

* More updates from code review

* Fix line length lint failure

* Apply suggestions from code review

Co-authored-by: Kevin Krsulich <kevin@krsulich.net>

* Expand sabre layout explanation

* Explain pass description in stage details header

Co-authored-by: Jake Lishman <jake@binhbar.com>
Co-authored-by: Kevin Krsulich <kevin@krsulich.net>
Co-authored-by: Kevin Hartman <kevin@hart.mn>
(cherry picked from commit 925f9f3)

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

Author: mergify[bot]