[DOCS] Creates custom landing page for the PHP client book

[szabosteve]

Overview

This PR adds a new landing page to the PHP client book so it will be more in line with the main docs landing page, the ES guide landing page, and the Kibana guide landing page.

• Current PHP docs landing page
• New PHP docs landing page proposed in this PR

The content can be easily modified based on feedback. I wanted to use the ElePHPant mascot in the hero image but the usage of the mascot and the logo is restricted. I need to figure out how and where to give credit to the creator and where to put the license text (these are the restrictions of the usage). I don't want to hold the restructuring back until that, so I used a temporary solution here.

[ezimuel]

@szabosteve thanks for the PR. I think it's better to use the official PHP logo. Regarding the license for using it's released under Creative Commons Attribution-Share Alike 4.0 International license.

[github-actions[bot]]

The backport to 8.7 failed:

The process '/usr/bin/git' failed with exit code 128

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-8.7 8.7
# Navigate to the new working tree
cd .worktrees/backport-8.7
# Create a new branch
git switch --create backport-1305-to-8.7
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 56e9abf561fd7502c736e8f9402f91225b063c1b
# Push it to GitHub
git push --set-upstream origin backport-1305-to-8.7
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.7

Then, create a pull request where the base branch is 8.7 and the compare/head branch is backport-1305-to-8.7.