fix(docs): add support for readthedocs

- Remove cmake variables from conf.py.in and rename it to confy.py
- Remove dependency on package plantuml
- Move tools/requirements.txt to docs
- Scale down image cca_software_arch.png

These changes make documentation build compatible when built using
cmake or readthedocs.

Signed-off-by: Arunachalam Ganapathy <arunachalam.ganapathy@arm.com>
Change-Id: I4331e6a435b25d1c50439ac85e396737d8042feb
diff --git a/docs/conf.py b/docs/conf.py
new file mode 100644
index 0000000..248b9c0
--- /dev/null
+++ b/docs/conf.py
@@ -0,0 +1,106 @@
+# -*- coding: utf-8 -*-
+# SPDX-License-Identifier: BSD-3-Clause
+# SPDX-FileCopyrightText: Copyright TF-RMM Contributors.
+#
+# Configuration file for the Sphinx documentation builder.
+#
+# See the options documentation at http://www.sphinx-doc.org/en/master/config
+
+import os
+import re
+from subprocess import check_output
+
+# -- Project information -----------------------------------------------------
+
+project = 'Realm Management Monitor'
+copyright = 'TF-RMM Contributors'
+author = 'TF-RMM Contributors'
+title = 'User Guide'
+
+try:
+  vregx = re.compile(r'tf-rmm-(?P<GIT_VERSION>v.+?)'
+                     r'(-[0-9]+-)?(?P<GIT_SHA>g[a-f0-9]{7,})?$')
+  git_result = check_output("git describe --tags --always",
+                            shell = True, encoding = 'UTF-8')
+  _v = vregx.match(git_result)
+  release = _v.group('GIT_VERSION')
+  if _v.group('GIT_SHA'):
+    version = release + "+" + _v.group('GIT_SHA')[:7]
+except:
+  version = 'Unknown'
+
+# -- General configuration ---------------------------------------------------
+
+# Add any Sphinx extension module names here, as strings. They can be
+# extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
+# ones.
+extensions = ['sphinx.ext.autosectionlabel', 'sphinxcontrib.plantuml']
+
+# Add any paths that contain templates here, relative to this directory.
+templates_path = ['_templates']
+
+# The suffix(es) of source filenames.
+source_suffix = ['.rst']
+
+# The master toctree document.
+master_doc = 'index'
+
+# The language for content autogenerated by Sphinx. Refer to documentation
+# for a list of supported languages.
+#
+# This is also used if you do content translation via gettext catalogs.
+# Usually you set "language" from the command line for these cases.
+language = None
+
+# List of patterns, relative to source directory, that match files and
+# directories to ignore when looking for source files.
+# This pattern also affects html_static_path and html_extra_path .
+exclude_patterns = []
+
+# The name of the Pygments (syntax highlighting) style to use.
+pygments_style = 'sphinx'
+
+# Load the contents of the global substitutions file into the 'rst_prolog'
+# variable. This ensures that the substitutions are all inserted into each page.
+with open('global_substitutions.txt', 'r') as subs:
+  rst_prolog = subs.read()
+
+# Minimum version of sphinx required
+needs_sphinx = '2.4'
+
+# -- Options for HTML output -------------------------------------------------
+
+# Don't show the "Built with Sphinx" footer
+html_show_sphinx = False
+
+# Show copyright info in the footer
+html_show_copyright = True
+
+# The theme to use for HTML and HTML Help pages.  See the documentation for
+# a list of builtin themes.
+html_theme = "sphinx_rtd_theme"
+
+# The logo to display in the sidebar
+html_logo = '_static/images/TrustedFirmware-Logo_standard-white.png'
+
+# Options for the "sphinx-rtd-theme" theme
+html_theme_options = {
+    'collapse_navigation': False, # Can expand and collapse sidebar entries
+    'prev_next_buttons_location': 'both', # Top and bottom of the page
+    'style_external_links': True # Display an icon next to external links
+}
+
+# Path to _static directory
+html_static_path = ['_static']
+
+# Path to css file relative to html_static_path
+html_css_files = ['css/rmm_custom.css',]
+
+# -- Options for autosectionlabel --------------------------------------------
+
+# Only generate automatic section labels for document titles
+autosectionlabel_maxdepth = 1
+
+# -- Options for plantuml ----------------------------------------------------
+
+plantuml_output_format = 'svg_img'