Adds links that point to the API contrib guidelines to repo docs

[szabosteve]

Overview

This PR adds links throughout the repository to the new API docs contribution guidelines. These guidelines serve as the source of truth for creating API documentation, consolidating accurate and up-to-date information from various sources (Confluence pages and internal documents).

Since most of the content in the elasticsearch-specification repo relates to defining API endpoints, linking to these guidelines provides helpful context for those who want to contribute to API docs.

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

No changes detected.

You can validate these APIs yourself by using the make validate target.

[pquentin]

Thanks! Since our contribution guide is already "CONTRIBUTING.md", I've tried to use a consistent but precise name for the new docs.

[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-5131-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 cdadc11ed3066e320241418f50500abdef24938e
# Push it to GitHub
git push --set-upstream origin backport-5131-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-5131-to-9.0.