fix(docs): resolve relative links by adding explicit .md suffix

Mkdocs treats bare directory URLs (`tests/`, `specs/reference/`) as
potential external references and emits INFO-level "unrecognized
relative link" messages, leaving the link text as-is. Suffixing with
`.md` or `index.md` lets mkdocs resolve the link cleanly and emit the
correct URL; the sibling cards in the home page already use this form.

Fixes:
- `tests/` -> `tests/index.md` in the home page card.
- `specs/reference/` -> `specs/reference/index.md` in the home page
  card and the literate nav entry (now resolves against the stub page
  added alongside the path rename).
- `../tests/prague/.../test_invalid/` -> `.../test_invalid.md` in
  `writing_tests/post_mortems.md`.

Author: danceratopz

docs/index.md

@@ -56,15 +56,15 @@ EELS is a collaborative effort between Ethereum Improvement Proposals (EIP) auth
 
     Browse the rendered Python specifications for the current fork and EIP.
 
-    [:octicons-arrow-right-24: Reference ↗](specs/reference/){target=_blank rel=noopener}
+    [:octicons-arrow-right-24: Reference ↗](specs/reference/index.md){target=_blank rel=noopener}
 
 - :material-format-list-checks: **Test Case Reference**
 
     ---
 
     Browse all test cases organized by fork and EIP.
 
-    [:octicons-arrow-right-24: Browse tests](tests/)
+    [:octicons-arrow-right-24: Browse tests](tests/index.md)
 
 </div>
 

docs/navigation.md

@@ -11,7 +11,7 @@
       * [Installation Troubleshooting](getting_started/installation_troubleshooting.md)
       * [Getting Help](getting_started/getting_help.md)
   * [Specifications](specs/index.md)
-      * [Reference ↗](specs/reference/)
+      * [Reference ↗](specs/reference/index.md)
       * [Writing Specs](specs/writing_specs.md)
       * [Adding a New EIP](specs/adding_a_new_eip.md)
       * [Spec Releases](specs/spec_releases.md)

docs/writing_tests/post_mortems.md

@@ -78,7 +78,7 @@ IDs of the tests added that now cover the missed scenario and link to the docume
 
 *Example:*
 
-- [`tests/prague/eip2537_bls_12_381_precompiles/test_bls12_g1msm.py::test_invalid\[fork_Prague-state_test---bls_g1_truncated_input-\]`](../tests/prague/eip2537_bls_12_381_precompiles/test_bls12_g1msm/test_invalid/)
+- [`tests/prague/eip2537_bls_12_381_precompiles/test_bls12_g1msm.py::test_invalid\[fork_Prague-state_test---bls_g1_truncated_input-\]`](../tests/prague/eip2537_bls_12_381_precompiles/test_bls12_g1msm/test_invalid.md)
 
 ### Framework/Documentation Changes