Update annotation documentation

Refs https://gitlab.com/arkindex/requests/-/issues/657

The annotate documents page is not up-to-date.

We should keep the existing structure (drawing shape, then creating elements and adding transcriptions)

Please update:

• the screenshots
• mention batch element creation
• transcriptions are edited inline (no modal)

You can keep:

• the element tree part
• the rotation & mirroring part
• the classification part