Add 8.18 documentation URLs to schema and OpenAPI

[miguelgrinberg]

Fixes #4369

The table.csv file now has a third column with the 8.18 documentation URL, if available. The mapping that was used comes from this spreadsheet.

These 8.18 docs URLs are imported in the new extLegacyDocUrl field of the schema. The OpenAPI generator then uses it to generate a x-legacyUrl extension inside the Documentation object. The name x-legacyUrl is made up by me, we can change it if there is a better suggestion, and in general, if there is a better word than legacy to use for this information let me know and I'll replace it.

[lcawl]

Output and table.csv LGTM

[swallez]

LGTM. Left a nitpick.

[github-actions]

The backport to 9.0 failed:

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

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-9.0 9.0
# Navigate to the new working tree
cd .worktrees/backport-9.0
# Create a new branch
git switch --create backport-4638-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 6f0fe001aa90f0f79da7eac081c62ceffee99e7b
# Push it to GitHub
git push --set-upstream origin backport-4638-to-9.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.0

Then, create a pull request where the base branch is 9.0 and the compare/head branch is backport-4638-to-9.0.