doc: Misc syntax and spelling fixes ...

Tidying up a few Sphinx warnings that had built-up over time.
None of these are critical but it cleans up the Sphinx output.

At the same time, fixing some spelling errors that were detected.

Change-Id: I38209e235481eed287f8008c6de9dedd6b12ab2e
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

doc: Render Marvell platform documents ...

The documentation for Marvell platforms was not included in the
rendered document output until now because, while it was mostly
valid RST format, the files were saved with a .txt extension.

This patch corrects some RST formatting errors, creates a document
tree (index page) for the Marvell documents, and adds the Marvell
subtree to the main index.

Change-Id: Id7d4ac37eded636f8f62322a153e1e5f652ff51a
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

doc: Move documents into subdirectories ...

This change creates the following directories under docs/
in order to provide a grouping for the content:

- components
- design
- getting_started
- perf
- process

In each of these directories an index.rst file is created
and this serves as an index / landing page for each of the
groups when the pages are compiled. Proper layout of the
top-level table of contents relies on this directory/index
structure.

Without this patch it is possible to build the documents
correctly with Sphinx but the output looks messy because
there is no overall hierarchy.

Change-Id: I3c9f4443ec98571a56a6edf775f2c8d74d7f429f
Signed-off-by: Paul Beesley <paul.beesley@arm.com>