doc: filesystems: proc: remove stale information from intro

Most of the information in the first paragraph of the
Introduction/Credits section is outdated.

Documentation update suggestions should go to documentation maintainers
listed in MAINTAINERS. Remove misleading contact information.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/cb4987a16ed96ee86841aec921d914bd44249d0b.1756294647.git.baruch@tkos.co.il