Clarify how-to docs for new units

[chiphogg]

Some users have found some aspects of these docs confusing. This PR
fixes them up to make the guide easier to use. Here are the problems
and how we handled them.

• Problem: It was unclear which namespace should contain the
  definitions of new units.

  • Solution: Append , in your project's namespace to the
    comments in the guide.

• Problem: It was unclear which namespace the symbols namespace
  should go in.

  • Solution: The above change should automatically clarify this
    as well.

• Problem: The au:: namespace was omitted from Au library
  constructs, making it unclear whether we had elided a using clause,
  or whether we wanted users to define the units in the au namespace.

  • Solution: The desired namespace should no longer be unclear.
    We had been writing the docs by omitting the au namespace to
    reduce visual clutter, but this is less appropriate for a how-to
    guide. We have added the namespace.

Of course, this last point applies with equal force to every other
how-to guide. It's OK to eliminate the au:: namespace qualifier in
the text, but we should keep the code samples as usable as possible.
Therefore, we apply this approach more consistently.

Fixes #504.

[Copilot]

Pull Request Overview

This PR clarifies the documentation for creating new units in Au by addressing namespace ambiguity and improving code sample usability. The changes ensure users understand where to define units and provide complete, runnable code examples.

• Added au:: namespace qualifiers to all code samples for clarity
• Clarified that new units should be defined in the user's project namespace
• Made code samples more self-contained and usable

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
docs/howto/new-units.md	Added namespace clarifications and au:: qualifiers to unit definition examples
docs/howto/quantity-template-parameters.md	Added au:: qualifiers to template parameter examples
docs/howto/new-dimensions.md	Added au:: qualifiers to dimension and unit creation examples
docs/howto/new-constants.md	Added namespace clarifications and au:: qualifiers to constant creation examples
docs/howto/forward-declarations.md	Added explanatory note about namespace usage and au:: qualifiers to code samples

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}