[Docs] Clarify usage of `echo` for adding extensions

Hey!

I noticed that the current documentation [here](https://github.com/asdf-community/asdf-php#note) suggests using the `echo {message} > {file}` command to add new extensions to PHP. There's a potential risk associated with it. Using the `>` operator will overwrite the entire file!

I recently lost all the configurations in my `php.ini` file when trying to add a new extension. While I understand that this was my fault, I believe we can improve the documentation to prevent such accidents in the future.

# Proposed Improvement:

I suggest updating the documentation to use the `>>` operator instead of `>`
Here's the revised command:
```bash
pecl install redis
pecl install imagick

echo "extension=redis.so
extension=imagick.so" >> $(asdf where php)/conf.d/php.ini
```

Open the documentation linked at the repository's #note section and compare its shell command with the proposed revision. Update the extension-installation example so it does not overwrite the existing php.ini configuration, then review the rendered documentation to confirm the command and warning are clear.

Written by the indexing model from the issue text.