Docs for creating a user to run Gitea on Fedora/RHEL/CentOS (#24725)

The current command doesn't work on Fedora/RHEL/CentOS.

- `--gecos`: use `--comment` instead.
- `--group`: use `--gid` instead, `--group` means "A list of
supplementary groups" on F/R/C.
- `--disabled-password`: leave `--password` empty instead, "The default
is to disable the password".
- `--home`: use `--home-dir` and `--create-home` instead.

docs/content/doc/installation/from-binary.en-us.md

@@ -80,6 +80,7 @@ git --version
 Create a user to run Gitea (e.g. `git`)
 
 ```sh
+# On Ubuntu/Debian:
 adduser \
    --system \
    --shell /bin/bash \
@@ -88,6 +89,17 @@ adduser \
    --disabled-password \
    --home /home/git \
    git
+
+# On Fedora/RHEL/CentOS:
+groupadd --system git
+adduser \
+   --system \
+   --shell /bin/bash \
+   --comment 'Git Version Control' \
+   --gid git \
+   --home-dir /home/git \
+   --create-home \
+   git
 ```
 
 ### Create required directory structure

docs/content/doc/installation/from-binary.zh-cn.md

@@ -74,6 +74,7 @@ git --version
 创建用户（推荐使用名称 `git`）
 
 ```sh
+# On Ubuntu/Debian:
 adduser \
    --system \
    --shell /bin/bash \
@@ -82,6 +83,17 @@ adduser \
    --disabled-password \
    --home /home/git \
    git
+
+# On Fedora/RHEL/CentOS:
+groupadd --system git
+adduser \
+   --system \
+   --shell /bin/bash \
+   --comment 'Git Version Control' \
+   --gid git \
+   --home-dir /home/git \
+   --create-home \
+   git
 ```
 
 ### 创建工作路径