Documentation/bpf: Use valid and new links in index.rst

There exists an error "404 Not Found" when I click the html link of
"Documentation/networking/filter.rst" in the BPF documentation [1],
fix it.

Additionally, use the new links about "BPF and XDP Reference Guide"
and "bpf(2)" to avoid redirects.

[1] https://www.kernel.org/doc/html/latest/bpf/

Fixes: d9b9170a2653 ("docs: bpf: Rename README.rst to index.rst")
Fixes: cb3f0d56e153 ("docs: networking: convert filter.txt to ReST")
Signed-off-by: Tiezhu Yang <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/index.rst

···
 This directory contains documentation for the BPF (Berkeley Packet
 Filter) facility, with a focus on the extended BPF version (eBPF).
 
-This kernel side documentation is still work in progress.  The main
+This kernel side documentation is still work in progress. The main
 textual documentation is (for historical reasons) described in
-`Documentation/networking/filter.rst`_, which describe both classical
-and extended BPF instruction-set.
+:ref:`networking-filter`, which describe both classical and extended
+BPF instruction-set.
 The Cilium project also maintains a `BPF and XDP Reference Guide`_
 that goes into great technical depth about the BPF Architecture.
 
···
 
 
 .. Links:
-.. _Documentation/networking/filter.rst: ../networking/filter.txt
+.. _networking-filter: ../networking/filter.rst
 .. _man-pages: https://www.kernel.org/doc/man-pages/
-.. _bpf(2): http://man7.org/linux/man-pages/man2/bpf.2.html
-.. _BPF and XDP Reference Guide: http://cilium.readthedocs.io/en/latest/bpf/
+.. _bpf(2): https://man7.org/linux/man-pages/man2/bpf.2.html
+.. _BPF and XDP Reference Guide: https://docs.cilium.io/en/latest/bpf/

Documentation/networking/filter.rst

···
 .. SPDX-License-Identifier: GPL-2.0
 
+.. _networking-filter:
+
 =======================================================
 Linux Socket Filtering aka Berkeley Packet Filter (BPF)
 =======================================================