The `.clang-format` configuration needs no changes to make the
version 18 format close to what version 15 produced before.

Update `.clang-format` with configuration to make the 6.0 format as
close as possible to what 3.8 produced before.  Then revise the style:

* Indent preprocessor directives (a feature new since 3.8)
* Add a newline and indentation before inheritance `:` and `,`

The `clang-format` tool does not format consistently across versions.
Specify the exact version we use to define formatting.

Replace the instructions that routed contributors to our old Wiki with
new instructions routing them to our GitLab instance.

Move it out of `CONTRIBUTING.rst`.

Document that we now define our coding style with `clang-format`.

Change-Id: I6dea8c3816f277b0e56e074ccd6c64a2cfeb094c

Change-Id: I5eb32dd2b8ca02800f15375dd4959c5c80d9ba7a

Also link to the file from the README.txt file.

Change-Id: I7dd74ce2a6d575a5503755d1aba2983907ccb7c9