Fix "undefined label" warnings caused by missing leading underscore

[cnk]

I saw the following warnings while building docs locally with make html:

writing output... [100%] topics/writing_templates
/Code/wagtail-dev/wagtail/docs/releases/1.11.rst:102: WARNING: undefined label: page_urls (if the link has no caption the label must precede a section header)
/Code/wagtail-dev/wagtail/docs/releases/1.6.1.rst:36: WARNING: undefined label: custom_page_managers (if the link has no caption the label must precede a section\
 header)
/Code/wagtail-dev/wagtail/docs/topics/permissions.rst:28: WARNING: undefined label: page_type_business_rules (if the link has no caption the label must precede a\
 section header)
generating indices... py-modindex done
writing additional pages... search done

Adding back the initial underscore in labels inside the eval_rst blocks make these error messages go away AND allow me to link to the page_type_business_rules section from the docs I was updating when I saw these warnings.

Before submitting, please review the contributor guidelines https://docs.wagtail.io/en/latest/contributing/index.html and check the following:

• Do the tests still pass? (https://docs.wagtail.io/en/latest/contributing/developing.html#testing) N/A
• Does the code comply with the style guide? (Run make lint from the Wagtail root). N/A
• For Python changes: Have you added tests to cover the new/fixed behaviour? N/A
• For front-end changes: Did you test on all of [Wagtail’s supported environments]. (https://docs.wagtail.io/en/latest/contributing/developing.html#browser-and-device-support)? N/A
  • Please list the exact browser and operating system versions you tested.
  • Please list which assistive technologies you tested.
• For new features: Has the documentation been updated accordingly? N/A

[squash-labs]

Manage this branch in Squash

Test this branch here: https://cnkfix-links-in-docs-r20my.squash.io

[zerolab]

LGTM, thank you @cnk