fix: Use URLs from existing policy template docs

[bsmth]

Description:

During conversion, I overlooked some of the links where we have parent-child relationships in the older docs (Certificates, SearchEngines). The main issue is that SanitizeOnShutdown was split into two pages without a parent that lists both types of policy.

This PR additionally changes some doc URLs to align with already-propagated ones so that redirection is easier.

Additions:

A parent page for SanitizeOnShutdown listing both policies.

Changes:

To align with existing URLs, I've made the following changes for convenience:

Policy name	Existing URL
Certificates	https://mozilla.github.io/policy-templates/#certificates
Certificates | ImportEnterpriseRoots	https://mozilla.github.io/policy-templates/#certificates--importenterpriseroots
Certificates | Install	https://mozilla.github.io/policy-templates/#certificates--install
SanitizeOnShutdown (Selective)	https://mozilla.github.io/policy-templates/#sanitizeonshutdown-selective
SanitizeOnShutdown (All)	https://mozilla.github.io/policy-templates/#sanitizeonshutdown-all
SearchEngines | Add	https://mozilla.github.io/policy-templates/#searchengines--add
SearchEngines | Default	https://mozilla.github.io/policy-templates/#searchengines--default
SearchEngines | PreventInstalls	https://mozilla.github.io/policy-templates/#searchengines--preventinstalls
SearchEngines | Remove	https://mozilla.github.io/policy-templates/#searchengines--remove
SecurityDevices	https://mozilla.github.io/policy-templates/#securitydevices
SecurityDevices (Deprecated)	https://mozilla.github.io/policy-templates/#securitydevices--deprecated

Related issues and pull requests:

Fixes #52