add docs about customizing pandoc (#4972)

closes #3288

doc/customizing-pandoc.md

@@ -0,0 +1,64 @@
+This document provides a quick overview over the various ways to
+customize pandoc's output. Follow the links to learn how to use each
+approach.
+
+[Templates](/MANUAL.html#templates)
+
+:   Pandoc comes with a template for (almost) every output format. A
+    template is a plain text file, that contains for example the line
+    `$body$`. That variable is replaced by the document's body text on
+    output.
+
+    There are many other variables, like `title`, `header-includes`,
+    etc. that are either set automatically, or that you can set using
+    [YAML metadata blocks](/MANUAL.html#extension-yaml_metadata_block),
+    [`--metadata`](/MANUAL.html#option--metadata) (which properly escape
+    things), or `--variable` (which does no escaping). You can also
+    generate your own template (e.g. `pandoc -D html > myletter.html`)
+    and customize that file, for example by introducing new variables.
+
+[reference.docx/pptx/odt](/MANUAL.html#option--reference-doc)
+
+:   To output a `docx`, `pptx` or `odt` document, which is a ZIP of
+    several files, things are a bit more complicated. Instead of a
+    single template file, you need to provide a customized
+    `reference.docx/pptx/odt`.
+
+[Lua filters](lua-filters.html) and [filters](filters.html)
+
+:   Templates are very powerful, but they are only a sort of scaffold to
+    place your document's body text in. You cannot directly change the
+    body text using the template (beyond e.g. adding CSS for HTML
+    output, or `\renewcommand` for LaTeX output).
+
+    If you need to affect the output of the actual body text, you
+    probably need a pandoc filter. A filter is a small program, that
+    transforms the document, between the parsing and the writing phase,
+    while it is still in pandoc's native format -- an abstract syntax
+    tree (AST), not unlike the HTML DOM. As can be seen in the [AST
+    definition](https://hackage.haskell.org/package/pandoc-types/docs/Text-Pandoc-Definition.html)
+    `Pandoc Meta [Block]`, a pandoc document is a chunk of metadata and
+    a list of `Block`s.
+
+    - There's a [list of third party filters on the
+      wiki](https://github.com/jgm/pandoc/wiki/Pandoc-Filters).
+    - Unless you have a good reason not to, it's best to write your
+      own filter in the Lua scripting language. Since pandoc ships
+      with a Lua interpreter, Lua filters are very portable and
+      efficient. See [Lua filters](lua-filters.html).
+    - For a gentle introduction into filters and writing them in any
+      programming language, see [filters](filters.html).
+
+Furthecustomizations
+
+:   - [Custom Styles in Docx](/MANUAL.html#custom-styles-in-docx)
+    - If you're converting from Markdown, see
+      - [Generic raw attributes](/MANUAL.html#generic-raw-attribute):
+        to include raw snippets
+      - [Divs and Spans](/MANUAL.html#divs-and-spans): generic blocks
+        that can be transformed with filters
+    - [Custom syntax highlighting](/MANUAL.html#syntax-highlighting),
+      provided by the [skylighting
+      library](https://github.com/jgm/skylighting)
+    - [Custom writers](/MANUAL.html#custom-writers)
+    - [Pandoc Extras wiki page](https://github.com/jgm/pandoc/wiki/Pandoc-Extras)

doc/filters.md

@@ -12,8 +12,8 @@ reader into pandoc’s intermediate representation of the
 document---an "abstract syntax tree" or AST---which is then
 converted by the writer into the target format.
 The pandoc AST format is defined in the module
-`Text.Pandoc.Definition` in
-[pandoc-types](https://hackage.haskell.org/package/pandoc-types).
+[`Text.Pandoc.Definition` in the `pandoc-types` package
+](https://hackage.haskell.org/package/pandoc-types/docs/Text-Pandoc-Definition.html).
 
 A "filter" is a program that modifies the AST, between the
 reader and the writer: