Broken link to `ASDF_CONFIG_FILE ` environment variable

[lindhe]

How can we improve the documentation?

Here, the docs refer to the ASDF_CONFIG_FILE environment variable:

https://github.com/asdf-vm/asdf/blob/ccdd47df9b73d0a22235eb06ad4c48eb57360832/docs/manage/configuration.md?plain=1#L57

That link is broken. On GitHub, the correct link seems to be #asdf_config_file while on the docs site it seems to be #asdf-config-file.

Considering the discrepancy, maybe it's best to just link to #environment-variables which seems to work the same on the web page and on GitHub?