Don't create .gitignore in pkg directory

[mrothNET]

Closes #728
Closes #984,
Closes #909
Closes #999.
Supersedes #1131, #985

Problematic Behaviour

Currently, wasm-pack generates a .gitignore file in the pkg directory during each build.

Suggested Solution

This PR proposes removing the automatic creation of the .gitignore file without introducing a replacement.

Justification

While many tools generate an appropriate .gitignore during project initialization, it’s relatively uncommon (in my experience) for build tools, like compilers, to create .gitignore files during a build.

Moreover, this behavior has led to several issues, as outlined in the following requests:

• Allow skipping .gitignore generation #728
• Don't overwrite an existing .gitignore file #984
• 'pkg/.gitignore' in deno target #909
• build --target web: have an option not to add .gitignore to pkg output folder #999

This PR aims to remove the generation of the problematic .gitignore file, addressing and potentially superseding these other PRs:

• Add skip-gitignore cli arg #1131
• Preserve existing .gitignore file in out directory #985

Changes made

• Remove one line from src/command/utils.rs
• Update documentation in docs/src/commands/build.md