Fix PGXS DOCS collision: use extension-specific filenames

[iemejia]

Fixes #33

Problem

PGXS installs files listed in DOCS to a flat shared directory:

/usr/share/doc/postgresql-doc-XX/extension/

With DOCS = $(wildcard *.md), both README.md and TESTS.md are
installed to this shared directory. The generic README.md filename
collides with any other extension that also declares DOCS = README.md
(e.g., postgres_protobuf, temporal_tables). When multiple extensions
are installed on the same system or composed via Docker COPY --from
layers, the last one silently overwrites the others.

Solution

Change DOCS to use a *.prefix.md wildcard pattern. Extension-specific
filenames are generated at build time by copying the source markdown files:

• README.md -> README.prefix.md
• TESTS.md -> TESTS.prefix.md

This way:

• Source files remain untouched (GitHub repo page still works)
• Installed doc files get unique names that cannot collide
• Generated files are added to .gitignore and cleaned by make clean

Changes

• Makefile: DOCS = $(wildcard *.prefix.md) + copy rules + clean target
• .gitignore: ignore the generated files