Prepare and release 0.7.0

[chalin]

For known issues with Docsy 0.7.0, see:

• https://github.com/google/docsy/issues/1568

Tasks

• [x] Mention known issues with Docsy 0.7.0 in the release comment, and link to the issue given above
• [x] Drop warning at the start of the README (#224)
• [x] Docsy user guide: Add to the clone command --depth 1 --branch v0.7.0, so that version v.7.0 is cloned (see corresponding pull request).

[chalin]

@geriom - might you be available to submit a PR for this in case @deining isn't available?

[deining]

All three tasks are completed, closing this issue.

[chalin]

Great thanks!