Update Concepts - Client Library - Execution Management (#356)

* restructuring of executor description, center alignment of figures

* added link to Executor Workshop, multi-threaded executor arxiv publication, minor typos

* typos

* re-ordered references in chronological order

* fixing links to ros2/rclc repo

* added figures for trigger condition

* moved all png to sub-folder /png, centering all png figures.

* added png files in /png subfolders to git

* corrected figure numbering

* fixed typo

* review Ralph (1): updated introduction

* removed motivation in section 'rclc Executor', added paragraph on multi-threaded executor.

* overall review

* restructured processing patterns, (LET as last pattern)

* minor improvements.

* minor improvements.

* Update _docs/concepts/client_library/execution_management/index.md

Co-authored-by: Ralph Lange <ralph-lange@users.noreply.github.com>

* Update _docs/concepts/client_library/execution_management/index.md

Co-authored-by: Ralph Lange <ralph-lange@users.noreply.github.com>

* Update _docs/concepts/client_library/execution_management/index.md

Co-authored-by: Ralph Lange <ralph-lange@users.noreply.github.com>

* Update _docs/concepts/client_library/execution_management/index.md

Co-authored-by: Ralph Lange <ralph-lange@users.noreply.github.com>

* second review from Ralph

* re-ordered example for embedded use-case (moved to the end)

* aligned and renamed section title of Examples to be the same as in section 'Analysis of processing patterns'

* typos

* fix broking internal link

* corrected link for cbg_executor_demo

* replaced links to cbg_executor source code from boschresearch repo to ros2/rclcpp repo

Co-authored-by: Ralph Lange <ralph-lange@users.noreply.github.com>

Author: JanStaschulat