ci: fix link checker config file name and broken URL (#41)

Rename .lychee.toml to lychee.toml to match lychee's default config
file name. Since lychee 0.18+, the default is 'lychee.toml' without
the dot prefix, so our .lychee.toml was silently ignored and the
medium.com exclusion never took effect.

Also fix typo in CONTRIBUTING.md: squidfun -> squidfunk (mkdocs-material
theme URL was returning 404).

Signed-off-by: Sebastien Tardif <sebtardif@ncf.ca>

Author: SebTardif

CONTRIBUTING.md

@@ -123,7 +123,7 @@ Commit the generated output.
 ## Documentation Site
 
 The `docs/` directory is configured as an [MkDocs](https://www.mkdocs.org/)
-site with the [Material](https://squidfun.github.io/mkdocs-material/) theme.
+site with the [Material](https://squidfunk.github.io/mkdocs-material/) theme.
 
 ### Local preview