Provide option to specify configuration file for `doc-changelog` action

### 📝 Description of the feature

For repositories containing a `pyproject.toml` and `towncrier.toml`, allow the user to specify using the `towncrier.toml` file. Currently, if both of these files exists it automatically defaults to the `pyproject.toml`

Requested by @ansjhenry

### 💡 Steps for implementing the feature

Add an optional input to the `doc-changelog` action to specify the name of the configuration file. By default, it will be `pyproject.toml`

### 🔗 Useful links and references

_No response_

Start at the doc-changelog action definition and inspect how its configuration file is selected when both pyproject.toml and towncrier.toml exist. Add an optional configuration-file input whose default remains pyproject.toml, and verify that specifying towncrier.toml selects that file.

Written by the indexing model from the issue text.