wrong links to constructors in pandoc lua filters web page; clarify that TableBody has no constructor

[massifrg]

I found a problem in the web page about Lua filters.

The links to constructors don't work, because they are made lowercase.
For example, in the Pandoc type section, there's a link to the pandoc constructor, whose href is pointing to:

https://pandoc.org/lua-filters.html#pandoc.pandoc

instead of

https://pandoc.org/lua-filters.html#pandoc.Pandoc

so the link doesn't work in the browser.

It would be also useful underlining, in the docs, that TableBody has no constructor and you instantiate one simply with a table having the attr (Attr), body (list of Row items), head (list of Row items) and row_head_columns (integer) fields.

(I apologize for not making a PR fixing these issues)

[wlupton]

I've been meaning to say something similar! Also note that the headers have apparently inappropriate "extension" info (see screenshot).

[jgm]

@tarleb will know how to fix these things.

[jgm]

I fixed the extension support links.

[tarleb]

Constructor links are fixed via 8c96c6e.

[jgm]

I pushed this to the website.