docs: fix LICENSE link in README to resolve myst.xref_missing

Replace the relative path (LICENSE) with an absolute URL so MyST
treats it as an external link rather than an internal cross-reference.
Removes the <!-- docs-end --> workaround so the License section is
now included in the rendered Sphinx docs as well.

Author: anevolbap

README.md

@@ -164,8 +164,6 @@ If you use CausalPy in your research, please cite it. A Zenodo DOI for stable re
 
 Plans for the repository can be seen in the [Issues](https://github.com/pymc-labs/CausalPy/issues).
 
-<!-- docs-end -->
-
 ## License
 
-[Apache License 2.0](LICENSE)
+[Apache License 2.0](https://github.com/pymc-labs/CausalPy/blob/main/LICENSE)

docs/source/index.md

@@ -6,7 +6,6 @@
 
 ```{include} ../../README.md
 :start-after: <!-- docs-start -->
-:end-before: <!-- docs-end -->
 ```
 
 :::{toctree}