[TASK] Establish a main "PHP architecture" chapter

[lolli42]

The patch merges "CGl->PHP architecture" and
"CGL->PHP best practices" to a general new top
level chapter. This should give extension developers general thoughts about separation of concerns and
best practices at hand, and documents how core
development handles specific PHP related architectural questions.

[github-actions]

The backport to 13.4 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 1420cd5f7... [TASK] Establish a main "PHP architecture" chapter
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

CONFLICT (modify/delete): Documentation/CodingGuidelines/CodingBestPractices/Index.rst deleted in 1420cd5f7 ([TASK] Establish a main "PHP architecture" chapter) and modified in HEAD.  Version HEAD of Documentation/CodingGuidelines/CodingBestPractices/Index.rst left in tree.
CONFLICT (file location): Documentation/CodingGuidelines/CodingBestPractices/ClassNamesOfUserFiles.rst added in HEAD inside a directory that was renamed in 1420cd5f7 ([TASK] Establish a main "PHP architecture" chapter), suggesting it should perhaps be moved to Documentation/PhpArchitecture/ClassNamesOfUserFiles.rst.

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-13.4 13.4
# Navigate to the new working tree
cd .worktrees/backport-13.4
# Create a new branch
git switch --create backport-5242-to-13.4
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 1420cd5f7ddecdd86cb9b9386bf82660418ba8e4,518a3879cb9c15109f86a53aff697137eebf6480
# Push it to GitHub
git push --set-upstream origin backport-5242-to-13.4
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-13.4

Then, create a pull request where the base branch is 13.4 and the compare/head branch is backport-5242-to-13.4.