Fixes two instances of kramdown warnings

What does this MR do?

While waiting on a links-related pipeline for an unrelated merge request (!117464 (merged)) I saw these Kramdown warnings flash by:

doc/development/sec/security_report_ingestion_overview.md

Malformed link:

kramdown warning(s) for <Nanoc::Core::CompilationItemRepView item.identifier=/ee/development/sec/security_report_ingestion_overview.md name=default>
  No link definition for link ID '#scan-runs-in-a-pipeline-for-a-non-default-branch' found on line 80

doc/architecture/blueprints/secret_detection/index.md

An attempt to use the [ ] syntax (👋🏻 @theoretick), which won't work. I found examples in other files in the doc/architecture directory of using ✓ for completed items, so I switched the list over to that format.

kramdown warning(s) for <Nanoc::Core::CompilationItemRepView item.identifier=/ee/architecture/blueprints/secret_detection/index.md name=default>
  No link definition for link ID 'x' found on line 166
  No link definition for link ID 'x' found on line 167
  No link definition for link ID ' ' found on line 168
  No link definition for link ID ' ' found on line 169
  No link definition for link ID ' ' found on line 170
  No link definition for link ID ' ' found on line 171
  No link definition for link ID ' ' found on line 172
  No link definition for link ID ' ' found on line 173
  No link definition for link ID ' ' found on line 174
  No link definition for link ID ' ' found on line 175
  No link definition for link ID ' ' found on line 176
  No link definition for link ID ' ' found on line 177
  No link definition for link ID ' ' found on line 178
  No link definition for link ID ' ' found on line 179
  No link definition for link ID ' ' found on line 180
  No link definition for link ID ' ' found on line 181
  No link definition for link ID ' ' found on line 182

Related issues

Author's checklist

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