Add User-defined wind documentation for InflowWind

[deslaughter]

This PR is ready to be merged

Feature or improvement description

This PR adds documentation for how to use the user-defined wind option in InflowWind. Comments and examples were added to the code and a section was added to the ReadTheDocs describing the process.

Related issue, if one exists

Impacted areas of the software

InflowWind modules/inflowwind and documentation docs

Additional supporting information

This PR was mostly generated by Claude Sonnet 4.5 and then reviewed and edited.

Generative AI usage
Co-authored-by: Anthropic Claude claude@anthropic.com

[Copilot]

Pull request overview

Adds developer-facing documentation and code templates to support implementing user-defined wind fields (WindType = 6) in InflowWind, including registry/type updates to carry user parameters and (optionally) time-series data.

Changes:

• Extends registry/type definitions for User_InitInputType and UserFieldType (including pack/unpack/copy/destroy support).
• Updates the WindType=6 code path to call UserField_GetVel() and improves template comments/error messages for user implementations.
• Adds new ReadTheDocs content for user-defined wind fields and links it into the InflowWind docs index (plus an internal markdown guide).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
modules/inflowwind/src/InflowWind_IO_Types.f90	Adds fields to User_InitInputType and updates copy/pack/unpack accordingly.
modules/inflowwind/src/InflowWind_IO.txt	Documents and registers the new User_InitInputType fields.
modules/inflowwind/src/InflowWind_IO.f90	Expands IfW_User_Init template and sets WindType=6 metadata/summary output.
modules/inflowwind/src/IfW_FlowField_Types.f90	Extends UserFieldType with optional arrays/filename and updates lifecycle + serialization routines.
modules/inflowwind/src/IfW_FlowField.txt	Documents and registers the new UserFieldType fields.
modules/inflowwind/src/IfW_FlowField.f90	Implements WindType=6 velocity evaluation loop via UserField_GetVel and adds detailed template docs.
modules/inflowwind/USER_DEFINED_WIND.md	Adds an implementation guide for developers working directly in the source tree.
docs/source/user/inflowwind/user_defined.rst	Adds RTD documentation page describing how to implement user-defined winds.
docs/source/user/inflowwind/index.rst	Adds the new user_defined.rst page to the toctree.
docs/source/user/inflowwind/appendix.rst	Adds download links intended to point to user-defined wind example files.

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