Description of Html2Text library import

Instructions
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1. Download release from https://github.com/mtibben/html2text/releases and unzip
2. Copy /path/to/html2text/src/Html2Text.php to lib/html2text/
3. Run: php totara/core/dev/fix_file_permissions.php --fix
4. Update lib/thirdpartylibs.xml

Local changes:
 * TL-34502 PHP 8.1 compatibility fixes