docs: update docs link from /doc to /docs (#771)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: SnO2WMaN <15155608+SnO2WMaN@users.noreply.github.com>
Co-authored-by: SnO₂WMaN <me@sno2wman.net>

Author: Copilot

README.md

@@ -1,4 +1,4 @@
-[Doc]: https://FormalizedFormalLogic.github.io/Foundation/doc
+[Docs]: https://FormalizedFormalLogic.github.io/Foundation/docs
 [Catalogue]: https://FormalizedFormalLogic.github.io/Catalogue
 [Zoo]: https://github.com/FormalizedFormalLogic/Zoo
 
@@ -8,7 +8,7 @@ Formalizing mathematical logic in Lean 4.
 
 ## Structure & Summary
 
-Main results of this repository. More detailed explanations are provided in the [Catalogue] and [Doc].
+Main results of this repository. More detailed explanations are provided in the [Catalogue] and [Docs].
 
 - `Propositional`: [Propositional Logic][prop]
   - [Tait-style Calculus][prop:classical_tait] and [Completeness][prop:classical_tait_complete]
@@ -73,7 +73,7 @@ Main results of this repository. More detailed explanations are provided in the
 ## Documents
 
 - [Catalogue], summary of results, _generated by [verso]_.
-- [Doc], catelogue of definitions and theorems, _generated by [doc-gen4](https://github.com/leanprover/doc-gen4)_.
+- [Docs], catelogue of definitions and theorems, _generated by [doc-gen4](https://github.com/leanprover/doc-gen4)_.
 - [Zoo], diagram of interrelationships among proof systems.
 
 [Monthly Reports]: https://formalizedformallogic.github.io/Foundation/book/Monthly-Reports