Enhance HTML/CSS of the generated ODF documentation

[svanteschubert]

With the update of the ODFDOM Java generation, the generation of the ODF documentation was updated as well, but the HTML/CSS format is still very bad, perhaps someone would like to give it a try!

Yesterday, I just spend an hour testing & updating a few things (but only updated the ODF 1.3 references).

The former/current way for ODF 1.2: https://tdf.github.io/odftoolkit/odf1.2/OpenDocument-v1.2-reference.html
A slightly updated documentation for ODF 1.3: https://tdf.github.io/odftoolkit/odf1.3/OpenDocument-v1.3-reference.html

If someone would like to give the Velocity Template a try:
https://github.com/tdf/odftoolkit/blob/odf-codegen/generator/schema2template/src/test/resources/test-input/odf/generation/odf-reference/odf-reference-template.vm

Velocity Syntax is pretty straightforward:
https://velocity.apache.org/engine/2.3/user-guide.html

The syntax is likely best viewed and edited by the Visual Studio Code extension - https://marketplace.visualstudio.com/items?itemName=Quidgest.vscode-velocity

In the OdfReference.html we might also link to the ODF elements & attributes description of the ODF specification (there is a fragment identifier pattern)

1. https://docs.oasis-open.org/office/OpenDocument/v1.3/os/part3-schema/OpenDocument-v1.3-os-part3-schema.html#element-table_table
2. https://docs.oasis-open.org/office/OpenDocument/v1.3/os/part3-schema/OpenDocument-v1.3-os-part3-schema.html#attribute-table_protected

and the RNG grammar that I have added to the standard as HTML:

1. http://docs.oasis-open.org/office/OpenDocument/v1.3/os/schemas/OpenDocument-v1.3-schema-rng.html#table-table

(just realized that only RNG definitions are being referenced, this can be improved and is not yet fully generated either, see https://github.com/oasis-tcs/odf-tc/blob/master/src/test/resources/odf1.3/tools/rng2html/HowToCreateRNG-HTML.md)

PS: I believe one VSCode Velocity plugin was able to indent the velocity files (but I wanted to postpone that till build & tests worked. If someone figures this out, we might switch the whitespace gobbling of Velocity in the generator)