Update security references

[jricher]

This change set updates a number of security-related discussions and inserts forward-links to the security considerations section.

• add security consideration anchors and MTLS discussions, closes #110
• add forward links for bearer token considerations from relevant portions, closes #289
• add forward references for impersonation and symmetric keys, closes #298 closes #289
• added considerations for phishing attacks, closes #255
• updated HTTPSig examples, closes #326

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