chore: normalize mixed casing of code block lang and theme parameters (#647)

exampleSite/content/en/features/code-blocks.md

@@ -21,21 +21,21 @@ To display an inline shortcode use single quotes:
 Code blocks can be uses without language specification:
 
 ````markdown
-```Plain
+```plain
 some code
 ```
 ````
 
 **Example:**
 
-```Plain
+```plain
 some code
 ```
 
 ... or if you need language specific syntax highlighting:
 
 ````markdown
-```Shell
+```shell
 # some code
 echo "Hello world"
 ```
@@ -43,7 +43,7 @@ echo "Hello world"
 
 **Example:**
 
-```Shell
+```shell
 # some code
 echo "Hello World"
 ```
@@ -55,7 +55,7 @@ Hugo has a build-in shortcode for syntax highlighting. To work properly with thi
 {{< tabs "uniqueid" >}}
 {{< tab "TOML" >}}
 
-```TOML
+```toml
 pygmentsUseClasses=true
 pygmentsCodeFences=true
 ```
@@ -63,7 +63,7 @@ pygmentsCodeFences=true
 {{< /tab >}}
 {{< tab "YAML" >}}
 
-```YAML
+```yaml
 pygmentsUseClasses: true
 pygmentsCodeFences: true
 ```

exampleSite/content/en/features/icon-sets.md

@@ -16,7 +16,7 @@ Regardless of which tool (or existing sprite) you choose, there are a few requir
 
 The result of a valid minimal SVG sprite file could look like this:
 
-```XML
+```xml
 <svg class="svg-sprite" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
     <symbol viewBox="-2.29 -2.29 28.57 28.57" id="arrow_back" xmlns="http://www.w3.org/2000/svg">
         <path d="M24 10.526v2.947H5.755l8.351 8.421-2.105 2.105-12-12 12-12 2.105 2.105-8.351 8.421H24z"/>
@@ -28,7 +28,7 @@ The result of a valid minimal SVG sprite file could look like this:
 
 FontAwesome provides three pre-build sprites included in the regular Web download pack, `sprites/brands.svg`, `sprites/regular.svg` and `sprites/solid.svg`. Choose your sprite to use and copy it to your projects root directory into `assets/sprites`, right beside your `content` folder:
 
-```Bash
+```bash
 my_projcet/
 ├── assets
 │   └── sprites

exampleSite/content/en/features/multilingual/_index.md

@@ -12,7 +12,7 @@ Hugo supports the creation of websites with multiple languages. In this post we
 
 You need to set a default language and configure at least two different languages used by your site to your configuration file at `config.toml`:
 
-```Toml
+```toml
 defaultContentLanguage = "en"
 
 [languages.en]
@@ -34,7 +34,7 @@ To customize translation strings used by the theme you can create a file `i18n/<
 
 For the [Bundle Menu](/usage/menus/#bundle-menu) as well as for the [Extra Header Menu](/usage/menus/#extra-header-menu) you can translate the name within the data file of the menu:
 
-```YAML
+```yaml
 ---
 more:
   # If `name` is a text, this text will be used as name for each language.