Better anchor URL for include reference

Hello,

When a ref is in a included file which is used multiple time, generated anchor it's always the same and not the best.

I created a [phpunit reproducer](https://github.com/doctrine/rst-parser/compare/0.5.x...alamirault:rst-parser:multi-reference-same-file). Problem was found on https://github.com/symfony/symfony-docs/pull/18791#discussion_r1307678134

The reproducer branch, has also a second commit, which "fix" the behavior. Please, tell me if I can open a PR, or if there is a better alternative

Start with the linked PHPUnit reproducer and compare the behavior between the 0.5.x branch and the multi-reference-same-file branch. Confirm how anchors are generated when one included file is reused, then verify that repeated references receive the intended distinct anchor URLs and that the reproducer passes with the proposed behavior.

Written by the indexing model from the issue text.