How to set a text as a footer with Pages?
The feature you are looking for are the footnotes/endnotes


  A document can contain footnotes, which appear at the bottom of the
  page, or endnotes, which appear at the end of the document or a
  section. You can’t have both footnotes and endnotes in the same
  document, but you can convert all notes in the document from one type
  to another.
  
  By default, footnotes and endnotes are numbered continuously
  throughout the document with Arabic numerals, but you can change this
  formatting.
  
  You can also restyle the text used within the note. For example, you
  can italicize or underline a book title within the note.
  
  Use the Footnotes pane of the Format inspector to format footnotes and
  endnotes.





  Insert a footnote, endnote, or section endnote
  The first time you
  insert a note, it’s a footnote by default. If you want document or
  section endnotes rather than footnotes, you must first add a footnote
  and then convert it to an endnote.
  
  
  Click in the text where you want to insert the symbol for a footnote
  or endnote.
  Click Insert in the toolbar, then choose Footnote.
  
  
  The symbol is inserted in the text and the insertion point moves to
  the footnote text field at the bottom of the page.
  
  
  Enter the footnote text.