Improve build script and instruction

[pan3793]

• Grant x permission to .dev/*.sh scripts so we can execute foo.sh directly instead of calling sh foo.sh
• Remove unused env var SPARK_WEBSITE_PATH in building instruction, it exists neither in this repo nor the main repo
• Correctly set JAVA_HOME for both x86_64 and arm platforms (is JAVA_HOME really required? I'm not sure)
• Fix issues when the host machine has the same UID and GID as the container. For example, my host runs Ubuntu 24.04, the default UID is 1000, and the docs image also uses Ubuntu 24.04 and has the user ubuntu with the same UID 1000.

Verification

• Intel CPU, OS Ubuntu 24.04, UID 1000, username chengpan

Before

$ sh .dev/build-docs.sh 
Successfully installed bundler-2.4.22
Parsing documentation for bundler-2.4.22
Installing ri documentation for bundler-2.4.22
Done installing documentation for bundler after 0 seconds
1 gem installed
Don't run Bundler as root. Installing your bundle as root will break this
application for all non-root users on this machine.
Bundle complete! 4 Gemfile dependencies, 33 gems now installed.
Bundled gems are installed into `./.local_ruby_bundle`
groupadd: GID '1000' already exists
useradd: UID 1000 is not unique
su: user docuser does not exist or the user entry does not contain all the required fields

After

$ .dev/build-docs.sh
Successfully installed bundler-2.4.22
Parsing documentation for bundler-2.4.22
Installing ri documentation for bundler-2.4.22
Done installing documentation for bundler after 0 seconds
1 gem installed
Don't run Bundler as root. Installing your bundle as root will break this
application for all non-root users on this machine.
Bundle complete! 4 Gemfile dependencies, 33 gems now installed.
Bundled gems are installed into `./.local_ruby_bundle`
groupadd: GID '1000' already exists
useradd: UID 1000 is not unique
Configuration file: /spark-website/_config.yml
            Source: /spark-website
       Destination: /spark-website/site
 Incremental build: disabled. Enable with --incremental
      Generating... 
                    done in 1.737 seconds.
 Auto-regeneration: disabled. Use --watch to enable.

• Also tested on a MacBook with Apple Silicon, works fine too.

$ .dev/build-docs.sh
Successfully installed bundler-2.4.22
Parsing documentation for bundler-2.4.22
Installing ri documentation for bundler-2.4.22
Done installing documentation for bundler after 0 seconds
1 gem installed
Don't run Bundler as root. Installing your bundle as root will break this
application for all non-root users on this machine.
Bundle complete! 4 Gemfile dependencies, 33 gems now installed.
Bundled gems are installed into `./.local_ruby_bundle`
groupadd: GID '20' already exists
useradd warning: docuser's uid 501 outside of the UID_MIN 1000 and UID_MAX 60000 range.
Configuration file: /spark-website/_config.yml
            Source: /spark-website
       Destination: /spark-website/site
 Incremental build: disabled. Enable with --incremental
      Generating...
                    done in 1.93 seconds.
 Auto-regeneration: disabled. Use --watch to enable.

[cloud-fan]

should this lead to a html file change?

[pan3793]

it's README.md, html does not have a counterpart