docs: crypto: convert api-intro.txt to ReST format

- Change title markups;
- Mark literal blocks;
- Use list markups at authors/credits;
- Add blank lines when needed;
- Remove trailing whitespaces.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/c71e2c73a787ec7814db09bec3c1359779785bfa.1592203650.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>