exporters/markdown_singlepage: make the "back to top" link anchor configurable

[nodiscc]

Ref. https://github.com/awesome-selfhosted/awesome-selfhosted-data/actions/runs/4642140620/jobs/8215750144

-**[`^        back to top        ^`](#awesome-selfhosted)**
+**[`^        back to top        ^`](#)**

For instance in https://github.com/awesome-selfhosted/awesome-selfhosted-data, tt should be set to #awesome-selfhosted)