Added APIs introduction

Added APIs introduction

Adding old api introduction page.

v2: Adding only 4.1 : Introduction

Bug-AGL: [SPEC-3633]

Signed-off-by: Shankho Boron Ghosh <shankhoghosh123@gmail.com>
Change-Id: Ibdabddccd73c1230c1b78c71c2cea4c5542b4a9b
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25558
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
(cherry picked from commit 2357897252c1fc3fd082113179bafb1e08b0d7ac)
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/25598