Expose batch drawing text example in the web doc

[pushfoo]

Closes #1571 (marked as 3.0 mandatory)

Changes

• Add the batch drawing example to the example gallery in the docs
• Clean up the example's docstrings and constant names
• Update the drawing_text and drawing_text_objects pages with cross-references

[pvcraven]

I like einarf's suggested improvements. Regardless, I think this is a good PR.

[einarf]

Let's just merge this!