Address review: remove note, reword title to avoid FeatureList false positive

Remove the explanatory RST comment note per @adiati98's request. A .. vale off
on the first line of the file does not suppress the Mautic.FeatureList finding
on 'assets' in the H1 title (reproduced on Vale 3.7.1 and 3.15.1), which is why
the note was priming it. Reword the title to 'Building CSS and frontend files'
so it no longer trips the rule, dropping the note and the ineffective top wrap.
All other generic 'assets' prose stays lowercase inside working vale off/on blocks.

Author: promptless[bot]

docs/development-environment/building_assets.rst

@@ -1,14 +1,5 @@
-.. vale off
-
-..
-
-   'assets' in the title below refers to compiled frontend build output, not the
-   Mautic Asset feature, so the Mautic.FeatureList substitution rule is suppressed here.
-
-Building CSS and frontend assets
-#################################
-
-.. vale on
+Building CSS and frontend files
+###############################
 
 Mautic uses SCSS for styling core UI Components and the SymfonyCasts Sass Bundle to compile them during development and for production builds.