enable markdown lint rules 12 and 31 (exercism#1191)

* enable rule

* resolve multiple blanks

* enable 31, apply lints

.markdownlint.yaml

@@ -22,10 +22,14 @@ MD003:
 MD009: true
 # no reversed links
 MD011: true
+# MD012/no-multiple-blanks https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md012
+MD012: true
 MD013: false
 # MD014/commands-show-output - Dollar signs used before commands without showing output
 MD014: false
 
+# MD031/blanks-around-fences https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md031---fenced-code-blocks-should-be-surrounded-by-blank-lines
+MD031: true
 # MD033/no-inline-html - Inline HTML
 MD033:
   # Allowed elements

concepts/destructuring/introduction.md

@@ -3,6 +3,7 @@
 Destructuring is a way to break apart compound values in Rust. It is often used to extract fields from tuples or structs.
 
 For example, the below snippet extracts two values from a tuple:
+
 ```rust
 let (first, second) = (1, 2);
 ```

concepts/numbers/introduction.md

@@ -3,16 +3,19 @@
 In Rust, there are are many specific types of numbers but they generally fall under one of two categories: integers or floating-point numbers.
 
 Initializing an integer:
+
 ```rust
 let x = 5; // rust defaults to an i32
 ```
 
 Initializing a large positive integer:
+
 ```rust
 let y: u64 = 7_000_000_000; // unsigned numbers can't be negative
 ```
 
 Initializing floating-point numbers:
+
 ```rust
 let x = 1.2; // defaults to f64
 

concepts/numeric-traits/about.md

@@ -2,4 +2,3 @@
 
 TODO
 - `Add`, `Sub`, `Mul`, `Div`
-

concepts/option/about.md

@@ -35,12 +35,14 @@ There are several common idioms for using the value which might be present in an
 ## Examples
 
 When a value exists in an `Option<u32>`:
+
 ```rust
 let has_something: Option<u32> = Some(100);
 assert_eq!(has_something.unwrap(), 100);
 ```
 
 When a value *does not* exist in an `Option<u32>`:
+
 ```rust
 let has_nothing: Option<u32> = None;
 assert_eq!(has_nothing, None);

concepts/string-vs-str/about.md

@@ -73,6 +73,7 @@ strings in no_std.)
 
 This section discusses when to use `String` or `&str` for struct fields.
 You'll often see structs defined with `String` instead of `&str` for field values.
+
 ```rust
 struct Record {
     latitude: f64,
@@ -82,6 +83,7 @@ struct Record {
     state: String,
 }
 ```
+
 By using `String`, the struct owns the data.
 This makes the code easier to read and teach for beginners.
 But it also means heap allocations will occur for each field.

concepts/structs/about.md

@@ -7,7 +7,6 @@ Structs collect a fixed, heterogeneous set of data into a single unit. Structs i
 - Every field of a struct has a particular name.
 - Visibility is tracked at the field level: each field may be independently public, private, or other[^1].
 
-
 # Structs Overview
 
 Structs are defined using the `struct` keyword, followed by the name of the type the struct is describing.

docs/CONTRIBUTING.md

@@ -25,7 +25,6 @@ Here are a few categories of welcome contribution:
 - updating documentation
 - fixing typos, misspellings, and grammatical errors
 
-
 ## Merging Philosophy
 
 A [pull request](https://docs.github.com/en/github/getting-started-with-github/github-glossary#pull-request) should address one logical change.

docs/maintaining.md

@@ -21,13 +21,16 @@ This is non-exhaustive.
 - Scripts should use `#!/usr/bin/env bash` as their shebang
   - This increases portability on NixOS and macOS because contributors' preferred bash may not be installed in `/bin/bash`.
 - Prefer snake case for script file names
+
     ```sh
     hello_world.sh
     ```
+
    - This simplifies development when upgrading a script into a proper language. *Rusty tooling anyone?*
 - Script file names should include the `.sh` extension
 - Set the executable bit on scripts that should be called directly.
 - Scripts should set the following options at the top
+
     ```bash
     set -eo pipefail
     ```