fix internal links

Pandoc does not care about input filenames, and thus the links to other
files in the book do not work.

What we can do is link to the autogenerated heading identifiers.  This
works even across multiple input files, because pandoc treats the input
as a single concatenated document.

Author: hmkemppainen

07-more-adverbs.md

@@ -53,7 +53,7 @@ When given two monadic functions on the left, `/` acts as a while:
 5
 ```
 
-The truthy and falsy values for while are the same as it is for If(`$`). You can rehash on what they are by checking [Part 3](03-functions-and-variables.md).
+The truthy and falsy values for while are the same as it is for If(`$`). You can rehash on what they are by checking [Part 3](#-if).
 
 While-scan is quite self-explanatory (It provides the intermediate results of a while loop):
 ```

10-io.md

@@ -16,7 +16,7 @@ The number of an input verb denotes is data format:
 - `1:` takes in arrays of bytes.
 
 <!-- The most commonly used I/O verbs are `0:` and `1:`. -->
-You have already seen `0:` used for output in the [first chapter](pt1.md).
+You have already seen `0:` used for output in the [first chapter](#introduction).
 
 Every I/O verb reads input when used monadically, and writes output when used dyadically. The general form is as follows:
 

12-thinking-in-k.md

@@ -140,7 +140,7 @@ Now *that* is matrix multiplication in K. This is the most direct algorithmic co
 matmul: {x{+/x*y}\:y}
 ```
 
-Now, we have a function which can be easily made tacit. With the rules from [Chapter 3](3-functions-and-variables.md#trains), we get our final result:
+Now, we have a function which can be easily made tacit. With the rules from [Chapter 3](#trains), we get our final result:
 
 ```
 matmul: (+/*)\: