libbpf: Rename libbpf documentation index file

This patch renames a documentation libbpf.rst to index.rst. In order
for readthedocs.org to pick this file up and properly build the
documentation site.

It also changes the title type of the ABI subsection in the
naming convention doc. This is so that readthedocs.org doesn't treat this
section as a separate document.

Signed-off-by: Grant Seltzer <grantseltzer@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210818151313.49992-1-grantseltzer@gmail.com

docs/index.rst

@@ -0,0 +1,22 @@
+.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+libbpf
+======
+
+For API documentation see the `versioned API documentation site <https://libbpf.readthedocs.io/en/latest/api.html>`_.
+
+.. toctree::
+   :maxdepth: 1
+
+   libbpf_naming_convention
+   libbpf_build
+
+This is documentation for libbpf, a userspace library for loading and
+interacting with bpf programs.
+
+All general BPF questions, including kernel functionality, libbpf APIs and
+their application, should be sent to bpf@vger.kernel.org mailing list.
+You can `subscribe <http://vger.kernel.org/vger-lists.html#bpf>`_ to the
+mailing list search its `archive <https://lore.kernel.org/bpf/>`_.
+Please search the archive before asking new questions. It very well might
+be that this was already addressed or answered before.