Add .gitleaksignore file to ignore sample tokens in documentation

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR introduces a .gitleaksignore file to ignore 2 known false-positive secrets from a documentation page.

Before:

$ gitleaks protect --no-banner --staged --redact --verbose
Finding:     --registration-token= "REDACTED"
Secret:      REDACTED
RuleID:      generic-api-key
Entropy:     4.563856
File:        doc/architecture/blueprints/runner_tokens/index.md
Line:        506
Fingerprint: doc/architecture/blueprints/runner_tokens/index.md:generic-api-key:506

6:04PM INF 1 commits scanned.
6:04PM INF scan completed in 59.1ms
6:04PM WRN leaks found: 1

After:

$ gitleaks protect --no-banner --staged --redact --verbose
6:08PM INF 0 commits scanned.
6:08PM INF scan completed in 58.3ms
6:08PM INF no leaks found

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