Split out lexicon, plugins and config page content

[mauriceyap]

In the documentation site, extract out the pre-templater content of the pages, lexicon.html, plugins.html and config.html into separate build targets, so that they can be used to build out a search index. In the search index, we do not want the content which the templater adds from template.html.

I have tested this by ensuring that these three generated templated pages before, and after these changes are the same:

$ cp $(plz build //docs:lexicon_html) tmp/lexicon_old.html
$ cp $(plz build //docs:config_html) tmp/config_old.html
$ cp $(plz build //docs:plugins_html) tmp/plugins_old.html

# Make the changes

$ cp $(plz build //docs:lexicon_html) tmp/lexicon_new.html
$ cp $(plz build //docs:config_html) tmp/config_new.html
$ cp $(plz build //docs:plugins_html) tmp/plugins_new.html

$ diff tmp/lexicon_old.html tmp/lexicon_new.html  # no output
$ diff tmp/config_old.html tmp/config_new.html # no output
$ diff tmp/plugins_old.html tmp/plugins_new.html # no output