Feature: Rest: .. include::: Support :start-after: and :end-before: in :literal: mode (#26130)

With this addition, we can include code samples in the docs using
comments as achors. This is analogous to mdBook's
[shiftinclude](https://github.com/daviddrysdale/mdbook-shiftinclude)
preprocessor, which is used extensively in the Status projects docs,
e.g.:
https://github.com/status-im/nim-chronos/blob/master/docs/src/tutorials/http_client/chapter1.md?plain=1#L16

P.S. One missing piece would be the ability to de-dent the included code
automatically but that's a feature for another PR. This isn't as
critical as the ability to include parts of the code.

lib/packages/docutils/rst.nim

@@ -3319,31 +3319,31 @@ proc dirInclude(p: var RstParser): PRstNode =
     rstMessage(p, meCannotOpenFile, filename)
   else:
     # XXX: error handling; recursive file inclusion!
+    let inputString = readFile(path)
+    let startPosition =
+      block:
+        let searchFor = n.getFieldValue("start-after").strip()
+        if searchFor != "":
+          let pos = inputString.find(searchFor)
+          if pos != -1: pos + searchFor.len
+          else: 0
+        else:
+          0
+
+    let endPosition =
+      block:
+        let searchFor = n.getFieldValue("end-before").strip()
+        if searchFor != "":
+          let pos = inputString.find(searchFor, start = startPosition)
+          if pos != -1: pos - 1
+          else: 0
+        else:
+          inputString.len - 1
+
     if getFieldValue(n, "literal") != "":
       result = newRstNode(rnLiteralBlock)
-      result.add newLeaf(readFile(path))
+      result.add newLeaf(inputString[startPosition..endPosition])
     else:
-      let inputString = readFile(path)
-      let startPosition =
-        block:
-          let searchFor = n.getFieldValue("start-after").strip()
-          if searchFor != "":
-            let pos = inputString.find(searchFor)
-            if pos != -1: pos + searchFor.len
-            else: 0
-          else:
-            0
-
-      let endPosition =
-        block:
-          let searchFor = n.getFieldValue("end-before").strip()
-          if searchFor != "":
-            let pos = inputString.find(searchFor, start = startPosition)
-            if pos != -1: pos - 1
-            else: 0
-          else:
-            inputString.len - 1
-
       var q: RstParser
       initParser(q, p.s)
       let saveFileIdx = p.s.currFileIdx