Fix challenge numbers and move to challenges/ dir (#22)

The challenges numbers in the index are being counted as a new chapter.
Instead convert "Challenges" into a chapter, and add a little blob for
the challenges chapter,
since mdbook requires all chapters to be a page.

doc/src/SUMMARY.md

@@ -12,6 +12,6 @@
 
 ---
 
-# Challenges
-- [Core Transmutation](./core-transmutation.md)
-- [Memory safety of core intrinsics](./intrinsics-memory.md)
+- [Challenges](./challenges.md)
+  - [Core Transmutation](./challenges/0001-core-transmutation.md)
+  - [Memory safety of core intrinsics](./challenges/0002-intrinsics-memory.md)

doc/src/challenges.md

@@ -0,0 +1,7 @@
+This chapter contains all existing challenges, including the ones that have already been solved or expired.
+
+For the most up-to-date information on a challenge status,
+please check their tracking issue linked in the challenge description.
+
+If you would like to submit a new challenge, please see
+["Call for Challenges" section](./general-rules.md#call-for-challenges).

doc/src/intrinsics-memory.md → doc/src/challenges/0002-intrinsics-memory.md

@@ -83,5 +83,5 @@ All proofs must automatically ensure the absence of the following undefined beha
 * Producing an invalid value
 
 
-Note: All solutions to verification challenges need to satisfy the criteria established in the [challenge book](general-rules.md)
+Note: All solutions to verification challenges need to satisfy the criteria established in the [challenge book](../general-rules.md)
 in addition to the ones listed above.

doc/src/template.md

@@ -1,6 +1,6 @@
 # Challenge XXXX[^challenge_id]: Challenge Title
 
-- **Status:** *One of the following: [Open | Resolved | Expired]*
+- **Status:** *One of the following: \[Open | Resolved | Expired\]*
 - **Solution:** *Option field to point to the PR that solved this challenge.*
 - **Tracking Issue:** *Link to issue*
 - **Start date:** *YY/MM/DD*