docs: add distributed mode limitations documentation

[1234-ad]

Description

This PR addresses issue #3203 by adding comprehensive documentation about the limitations of using -f - (distributed locustfile) with multiple files.

Changes

• Added new documentation file docs/distributed-limitations.rst
• Clearly explains why -f - doesn't work with multiple files
• Provides practical workarounds for multi-file test structures
• Includes examples of both problematic and working approaches

Why this change?

Issue #3203 highlighted that the current documentation mentions -f - works for "single locustfiles" but doesn't clearly explain the implications for users with multi-file test structures. This leads to confusion and ModuleNotFoundError when users try to import from other modules.

Benefits

• Prevents user frustration - Clear upfront explanation of limitations
• Provides solutions - Multiple workaround strategies
• Improves developer experience - Users can make informed architectural decisions
• Reduces support burden - Fewer duplicate issues about import errors

Documentation Structure

The new file covers:

1. Clear explanation of the single-file limitation
2. Examples of what doesn't work and why
3. Multiple workaround strategies
4. Example of proper single-file structure

This addresses the maintainer's comment that "serving multiple files is a bit thorny" by documenting the current limitations while providing practical alternatives.

[cyberw]

It is nice that you provide all this info but it is a little too verbose. Specifically the code example is not necessary, and I think it something only <1% of locust users will care about so it doesnt deserve its own top level page in the docs.

Maybe add it under the FAQ and link it from the note that is already there next to the introduction of -f - ("The -f - argument tells Locust to get the locustfile from master instead of from its local filesystem. This only works for single locustfiles.")