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 <[email protected]>

* 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 <[email protected]>

* Fix trailing whitespace

* Expand init stage definition

* Apply suggestions from code review

Co-authored-by: Kevin Krsulich <[email protected]>

* 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 <[email protected]>

* Update qiskit/transpiler/preset_passmanagers/__init__.py

Co-authored-by: Kevin Hartman <[email protected]>

* More updates from code review

* Fix line length lint failure

* Apply suggestions from code review

Co-authored-by: Kevin Krsulich <[email protected]>

* Expand sabre layout explanation

* Explain pass description in stage details header

Co-authored-by: Jake Lishman <[email protected]>
Co-authored-by: Kevin Krsulich <[email protected]>
Co-authored-by: Kevin Hartman <[email protected]>
(cherry picked from commit 925f9f3)

Co-authored-by: Matthew Treinish <[email protected]>