Add documentation project overview

OVERVIEW.md

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+# The PHP Documentation
+
+The PHP documentation is built from XML files and render into PHP files
+that are available on the [php.net/manual](https://php.net/manual/en) pages.
+All parts of the process live in the following GitHub repositories:
+
+- [**doc-\* repos**](#the-english-language-documentation-and-its-translations):
+   these are the repositories where all the information on documentation pages live
+   and this is where changes to the wording and/or structure
+   of the documentation's content can be made
+- [**doc-base**](#doc-base): validates and pre-processes the language files
+- [**PhD**](#phd): renders pages into .php files which include information on the files
+   and function calls to include the documentation page headers and footers
+- [**web-php**](#web-php): serves up the .php files generated by PhD
+   and includes the functions to generate page headers and footers
+   (to include contribution call-out and user notes)
+
+## The English Language Documentation And Its Translations
+
+These repositories contain PHP's documentation in their respective languages
+and these are where changes to the content/structure
+of the [php.net's documentation pages](https://www.php.net/docs.php) are made.
+
+Please refer to [doc-base's](https://github.com/php/doc-base)
+[contribution guidlines](https://github.com/php/doc-base/CONTRIBUTING_DOCS.md)
+on how to contribute to the documentation.
+
+## doc-base
+
+[doc-base](https://github.com/php/doc-base) validates and combines the documentations' XML files.
+It includes a skeleton/main table of contents of the PHP documentation (index.php)
+and the `configure.php` script which processes the documentation:
+- pulls version info file from repo files
+- starting from the skeleton/main table of contents file, it iterates over all files in the language repository and includes all elements with the appropriate IDs
+- pulls modification history file
+- generates the file entities from the source language directory and writes these back into the same directory
+- loads the combined XML file into memory
+- runs/executes all XInclude/XPointer links
+- subtitutes entities
+- validates the document based on its DTD
+
+Please refer to [doc-base's](https://github.com/php/doc-base)
+[contribution guidlines](https://github.com/php/doc-base/CONTRIBUTING_DOCS.md)
+on further details on how to use doc-base.
+
+## PhD
+
+[PhD](https://github.com/php/phd) (PHP DocBook) renders the combined XML file,
+the version file and modification history files generated/copied by [doc-base](https://github.com/php/doc-base)
+into the final PHP documentation pages.
+
+While PhD is capable of generating the documentation in a number of formats,
+the PHP documentation website is using the format that generates .php files.
+This format adds PHP code in each file that contains information on the current page
+(e.g. title, breadcrumbs, source file name/path, last modified date/time, contributor list)
+and calls to setting up the header and footer for the page.
+
+It's important to note that the function calls included in the pages here
+are for functions that are defined in [web-php](https://github.com/php/web-php).
+A new functionality added to [web-php](https://github.com/php/web-php) can only be used
+if the appropriate format in PhD is modified to include the function call.
+
+Other than the above PhD also does the following:
+- writes the search-index and search-description json files
+- copies (by default downloads) css files into the output directory
+- copies images to the images directory
+- generates and saves the TOC (table of contents) files
+ (the TOC/navbar on the right side of the documentation pages)
+
+Please refer to [doc-base's](https://github.com/php/doc-base)
+[contribution guidlines](https://github.com/php/doc-base/CONTRIBUTING_DOCS.md)
+on further details on how to use PhD.
+
+## web-php
+
+[web-php](https://github.com/php/web-php) is the repository that PHP's web servers actually run
+(ie. almost everything accesible on [php.net](https://php.net)).
+
+This repository includes the functions that generate the header (top navigation and search bar)
+and footer (copyright, privacy policy, etc.) for most php.net pages
+and it also includes the functions that generate the additional header (breadcrumbs, language dropdown, etc.)
+and footer (contribution and user notes sections) of documentation pages.
+
+It uses the css, image, TOC and search index/description files copied/generated by PhD
+to serve up images/css files, generate the right side TOC/navbar on the documentation pages
+and make the search bar on [php.net](https://php.net) work.
+
+Please refer to [doc-base's](https://github.com/php/doc-base)
+[contribution guidlines](https://github.com/php/doc-base/CONTRIBUTING_DOCS.md)
+on further details on how to use web-php.