Add markdownlint (#20512)

Add `markdownlint` linter and fix issues. Config is based on the one
from electron's repo with a few rules relaxed.

docs/content/doc/features/authentication.en-us.md

@@ -203,7 +203,7 @@ configure this, set the fields below:
 
 - Force SMTPS
 
-  - SMTPS will be used by default for connections to port 465, if you wish to use SMTPS 
+  - SMTPS will be used by default for connections to port 465, if you wish to use SMTPS
   for other ports. Set this value.
   - Otherwise if the server provides the `STARTTLS` extension this will be used.
 

docs/content/doc/features/comparison.zh-cn.md

@@ -109,7 +109,6 @@ _表格中的符号含义:_
 | Pull/Merge requests 模板             | ✓     | ✓    | ✓         | ✓         | ✓         | ✘         | ✘            |
 | 查看 Cherry-picking 的更改           | ✘     | ✘    | ✘         | ✓         | ✓         | ✘         | ✘            |
 
-
 #### 第三方集成
 
 | 特性                       | Gitea | Gogs | GitHub EE | GitLab CE | GitLab EE | BitBucket | RhodeCode CE |

docs/content/doc/features/localization.en-us.md

@@ -26,6 +26,8 @@ For changes to a **non-English** translation, refer to the Crowdin project above
 
 Any language listed in the above Crowdin project will be supported as long as 25% or more has been translated.
 
-After a translation has been accepted, it will be reflected in the main repository after the next Crowdin sync, which is generally after any PR is merged.  
-At the time of writing, this means that a changed translation may not appear until the following Gitea release.  
+After a translation has been accepted, it will be reflected in the main repository after the next Crowdin sync, which is generally after any PR is merged.
+
+At the time of writing, this means that a changed translation may not appear until the following Gitea release.
+
 If you use a bleeding edge build, it should appear as soon as you update after the change is synced.

docs/content/doc/features/localization.zh-tw.md

@@ -25,6 +25,8 @@ menu:
 
 上述 Crowdin 專案中列出的語言在翻譯超過 25% 後將被支援。
 
-翻譯被認可後將在下次 Crowdin 同步後進入到主儲存庫，通常是在任何合併請求被合併之後。  
-這表示更改的翻譯要到下次 Gitea 發佈後才會出現。  
+翻譯被認可後將在下次 Crowdin 同步後進入到主儲存庫，通常是在任何合併請求被合併之後。
+
+這表示更改的翻譯要到下次 Gitea 發佈後才會出現。
+
 如果您使用的是最新建置，它將會在同步完成、您更新後出現。