How to Display the Indice Page Numbers in Bold?

From the makeindex documentation (section 2.2 The Basics, p 5):

In some indexes, certain page numbers are specially formatted - for example, an italic page number may indicate the primary reference, and an n after a page number may denote that the item appears in a footnote on that page. MakeIndex makes it easy to format an individual page number any way you want. For any string of characters σ, the command index{...|σ} produces a page number of the form σ{n}. Similarly, the command index{...|(σ} may produce a page number of the form σ{n–m}.

So, to make an individual entry's page number appear bold, use index{Heading@!Entry-1|textbf}. To make all page numbers appear bold, you could do this:

documentclass{article}

usepackage{imakeidx}
makeindex
usepackage{idxlayout}

% Make all page numbers in index bold
letoldindexindex
renewcommand{index}[1]{%
  oldindex{#1|textbf}%
}%

begin{document}

newpage Another sentenceindex{Heading@!Entry-1}
newpage Another sentenceindex{Heading@!Entry-2}
newpage Another sentenceindex{Heading@!Entry-3}

idxlayout{columns=1}
printindex

end{document}