doc: fix syntax highlighting in code blocks

- remove json syntax highlighting from invalid json block
- fix warning about unknown lexer `conf`
- fix syntax highlighting for example htaccess

doc/md/Reverse-proxy.md

@@ -68,7 +68,7 @@ See also [proxy-related](https://github.com/shaarli/Shaarli/issues?utf8=%E2%9C%9
 ## HAProxy
 
 
-```conf
+```
 global
     [...]
 

doc/md/Shaarli-configuration.md

@@ -22,7 +22,7 @@ Some settings can be configured directly from a web browser by accesing the `Too
 
 ## Configuration file example
 
-```json
+```
 <?php /*
 {
     "credentials": {

doc/md/Troubleshooting.md

@@ -40,7 +40,7 @@ Shaarli redirections will not work properly. To solve this, assign a local domai
 
 - On hosts (such as **free.fr**) which only support PHP 5.6, Shaarli [v0.10.4](https://github.com/shaarli/Shaarli/releases/tag/v0.10.4) is the maximum supported version. At the root of your webspace create a `sessions` directory and a `.htaccess` file containing:
 
-```xml
+```apacheconf
 <IfDefine Free>
 php56 1
 </IfDefine>