doc: Reword document titles

This patch attempts to standardise the document titles as well as
adding titles to documents that were missing one. The aim is to
remove needless references to "TF-A" or "Trusted Firmware" in the
title of every document and to make sure that the title matches
with the document content.

Change-Id: I9b93ccf43b5d57e8dc793a5311b8ed7c4dd245cc
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

docs/process/coding-guidelines.rst

-Trusted Firmware-A Coding Guidelines
-====================================
-
-
+Coding Style & Guidelines
+=========================
 
 .. contents::
 

docs/process/contributing.rst

-Contributing to Trusted Firmware-A
-==================================
+Contributor's Guide
+===================
 
 Getting Started
 ---------------

docs/process/index.rst

@@ -7,7 +7,7 @@ Processes & Policies
    :numbered:
 
    release-information
-   security-center
+   security
    platform-compatibility-policy
    coding-guidelines
    contributing

docs/process/platform-compatibility-policy.rst

-TF-A Platform Compatibility Policy
-==================================
+Platform Compatibility Policy
+=============================
 
 
 

docs/process/release-information.rst

-TF-A Release Information
-========================
+Release Processes
+=================
 
 .. section-numbering::
     :suffix: .

docs/process/security-center.rst → docs/process/security.rst

-Security Center
-===============
+Security Handling
+=================
 
 Security Disclosures
 --------------------