Docs: HF-24 add .md extension to bare slug links in built-in-functions

Harness 04-link-validation flagged two relative links missing the .md
extension: localizing-functions → localizing-functions.md and
custom-functions → custom-functions.md. VuePress resolves both forms
at build time, but adding .md is consistent with all other links in
the file and removes the harness warning.

Author: marcin-kordas-hoc

docs/guide/built-in-functions.md

@@ -22,7 +22,7 @@ spreadsheet software. That is because a spreadsheet is probably the most
 universal software ever created. We wanted the same flexibility for HyperFormula
 but without the constraints of the spreadsheet UI.
 
-Each of HyperFormula's built-in function names is available in [17 languages](localizing-functions.md#list-of-supported-languages) and [custom language packs](localizing-functions) can be added.
+Each of HyperFormula's built-in function names is available in [17 languages](localizing-functions.md#list-of-supported-languages) and [custom language packs](localizing-functions.md) can be added.
 
 The latest version of HyperFormula has an extensive collection of
 **{{ $page.functionsCount }}** functions grouped into categories:
@@ -44,7 +44,7 @@ The latest version of HyperFormula has an extensive collection of
 _Some categories such as compatibility and cube are yet to be supported._
 
 ::: tip
-You can modify the built-in functions or create your own, by adding a [custom function](custom-functions).
+You can modify the built-in functions or create your own, by adding a [custom function](custom-functions.md).
 :::
 
 ## List of available functions