[FEATURE] Create a page about site packages

[linawolf]

Move advanced introduction texts from the sitepackage tutorial. Move the sitepackage builder from the Kickstarter section.task/sitepackage

Link here instead of the tutorial when mentioning site packages

Releases: main, 12.4

[garvinhicking]

Only a quick look, but this looks good to me. If there's anything we can improve on it later, so fine fore me to merge as-is!

[github-actions]

The backport to 12.4 failed:

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

stderr

error: could not apply f94ba8a00... [FEATURE] Create a page about site packages
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

Auto-merging Documentation/ApiOverview/ContentElements/ContentElementsWizard.rst
CONFLICT (content): Merge conflict in Documentation/ApiOverview/ContentElements/ContentElementsWizard.rst
Auto-merging Documentation/ApiOverview/ContentElements/Index.rst
Auto-merging Documentation/ExtensionArchitecture/HowTo/ExtendExtbaseModel/Index.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-12.4 12.4
# Navigate to the new working tree
cd .worktrees/backport-12.4
# Create a new branch
git switch --create backport-4807-to-12.4
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick f94ba8a0053e39a76d3d89ca707d367f0578820d
# Push it to GitHub
git push --set-upstream origin backport-4807-to-12.4
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-12.4

Then, create a pull request where the base branch is 12.4 and the compare/head branch is backport-4807-to-12.4.