+ use luaL_ref/luaL_unref to store lua scalar instead lua metatable.
+ add :is_closed method.

    1. Unified blurb for every site page
    2. remove tnt-html.xsl.cmake tnt-html-chunk.xsl.cmake in favor of
    --stringparam and -param flags of xsltproc and saxon
    3. Create flags for input/output in static.py script for OOS build

All layouts are in doc/www-data.in/_layout and are written in Jinja1
(django-like templates). All texts are in doc/www-data.in/_texts

Text files contains YAML files with chunks of markdown strings:
```
$module:
    $uno: |
        some text in markdown
    $dos: |
        it may be multiline
    $tres: |
        # header 1
        ```
        may have some embedded code
        ```
        e.t.c
    ...
```
where `$module` is name of layout file, `$uno`, `$dos`, `$tres`... are
name of parts (template parts `{{ $module.$part }}`)
Site still stores files in `doc/www-data` for copying on server.
Also it integrates documentation (only on stable/master branches) into site (still templlates)
Usage of new `extra/static.py`:
```bash
$ cmake . -DENABLE-DOC=TRUE
$ make
$ cd /doc/www-data.in/
$ ../../extra/static.py site `# build site`
$ ../../extra/static.py docs `# embed docs`
```

 ### documentation building

Add `html-saxon-chunk` and `html-chunk` targets for building chunked
html documentation.

`make` in `doc/user` now builds `html-saxon` and `html-saxon-chunk`
targets

> remove script/ directory and move it's content to extra/