[DOCS] Add redirect for field aliases (#77349) (#77358)

PRs #73062 and #73043 repurposed the `alias` anchor for a new guide for index
and data stream aliases. Previously, this anchor was used for our field alias
documentation.

Repurposing the anchor has caused continuity errors for users selecting
different versions of the ES docs. It could also cause confusion for users with
a `/current/` link to the `alias` page.

This adds a redirect page for the `alias` anchor.

It also fixes a bread crumb issue for redirects following the 'Modifying your
Data' redirect page.

docs/reference/redirects.asciidoc

@@ -3,6 +3,11 @@
 
 The following pages have moved or been deleted.
 
+[role="exclude",id="alias"]
+=== Field aliases
+
+Refer to <<field-alias>>.
+
 [role="exclude",id="grok-basics"]
 === Grok basics
 
@@ -399,7 +404,7 @@ See <<getting-started-index>>.
 See <<indices-delete-index>>.
 
 [role="exclude", id="getting-started-modify-data"]
-== Modifying your data
+=== Modifying your data
 See <<docs-update>>.
 
 [role="exclude", id="indexing-replacing-documents"]