Fix some internal links

[adriendupuis]

Question	Answer
JIRA Ticket
Versions
Edition

Fix the following errors on 4.5:

INFO    -  Doc file 'api/rest_api/rest_api_usage/rest_api_usage.md' contains an unrecognized relative link 'rest_requests#siteaccess', it was left as is. Did you mean 'rest_requests.md#siteaccess'?
INFO    -  Doc file 'customer_management/create_user_registration_form.md' contains an unrecognized relative link 'user_registration#user-types', it was left as is.
INFO    -  Doc file 'infrastructure_and_maintenance/clustering/clustering.md' contains an unrecognized relative link 'reverse_proxy.md/#using-varnish-or-fastly', it was left as is.
INFO    -  Doc file 'resources/phpstorm_plugin.md' contains an unrecognized relative link 'install_on_ibexa_cloud', it was left as is.

Checklist

• Text renders correctly
• Text has been checked with vale
• Description metadata is up to date
• Redirects cover removed/moved pages
• Code samples are working
• PHP code samples have been fixed with PHP CS fixer
• Added link to this PR in relevant JIRA ticket or code PR