a few more cleanup things

[michael-petersen]

I was overambitious in merging -- I just noticed a few more things that could be updated for navigability in the top-level directory. Most of these come from helpful comments provided by @adrn -- thanks!

[Copilot]

Pull Request Overview

This PR performs minor cleanups to improve navigability in the top-level directory by updating documentation and removing obsolete files. Key changes include an updated README table structure and descriptions, consolidation of installation instructions into INSTALL.md, and removal of outdated files such as INSTALL and .gitattributes.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
README.md	Updated repository organization table and documentation text
INSTALL.md	Added a pointer to the up-to-date installation guide
INSTALL	Removed redundant installation instructions
.gitattributes	Removed outdated Git attributes; confirm line-ending handling strategy

Comments suppressed due to low confidence (2)

.gitattributes:1

• Removing .gitattributes may affect the repository's line-ending normalization across different OS environments. Please confirm that Git configurations or alternative strategies are in place to maintain consistent line endings.

-* text=auto !eol

INSTALL:1

• The removal of the INSTALL file is acceptable since the installation instructions have been moved to INSTALL.md. Ensure that any remaining references to this file in documentation or build scripts are updated accordingly.

-See the installation guide in the documentation for the most up-to-date instructions on

[The9Cat]

expui is mostly used by standalone stuff and pybind11. But exp uses some of the coefficient classes and the new OutVel implementation also uses FieldBasis. We could say something like:

Classes used by Python wrappers and by the nbody code and other standalone utilities for coefficient IO support.