Docs: make source file path references consistent

Hi Hackers,

This patch is derived from [1]. While reviewing that thread, I noticed that
when the documentation refers to .h and .cfiles in the PostgreSQL source
tree, it uses a mix of full source-tree paths and short relative paths.
This inconsistency can be confusing for future documentation authors, who
may be unsure which form to use.

This patch aims to make the usage consistent. When the documentation
explicitly refers to a file’s location in the source tree (for example, “in
the file …”, "see the file ..."), it uses the full path. When the
documentation refers to a header in the context of inclusion, it uses the
short relative path, matching the form used in #include <header> directives.

[1]
https://postgr.es/m/CAHGQGwEyYUzz0LjBV_fMcdwU3wgmu0NCoT+JJiozPa8DG6eeog@mail.gmail.com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/