The ng-container and binding template guide files each began with a
UTF-8 BOM (EF BB BF) before the leading `#`. The docs markdown parser
only promotes `#` to an H1 when it is the first character on the line,
so the BOM demoted the title to paragraph text (`<p># ...</p>`) and the
standard docs header (breadcrumbs, page title, edit button) never
rendered.

Stripping the BOM restores `#` as the first character, so both pages
now generate the proper `<header class="docs-header">` block. Verified
by rebuilding //adev/src/content/guide/templates:templates and
inspecting the generated HTML.

Fixes angular#69889

…h a BOM

A leading UTF-8 byte order mark (U+FEFF) before the first `#` stops the
Markdown parser from recognizing the heading, so the guide renders its
title as a paragraph and drops the standard docs header. The character
is invisible, so it cannot be caught in review.

Add a check in the guides generation pipeline that throws when a source
file starts with a BOM, failing the build with the offending file name.
This sits alongside the existing unknown-anchor check and prevents the
regression fixed in angular#69889 from recurring.