docs: webhook documentation missing / unclear

[matthewelwell]

The Flagsmith platform provides 3 different types of webhook:

• Identity webhooks (triggered when an identity evaluates their flags, useful for analytics processing)
• Environment webhooks (triggered when flags change in an environment, useful for custom caching behaviuor)
• Audit log webhooks (triggered whenever an Audit Log record is written, useful for security purposes)

Previously, these were documented individually, however, since the documentation update it seems as though they have been condensed / muddled up.

Using the wayback machine, we can see the documentation for each individual webhook here:

• Identity
• Environment
• Audit Log

Since the documentation updates, we only have a page on the identity webhooks here (which also randomly includes information part-way through around the audit log webhooks), and the Audit Log webhooks here (which no longer includes the payload information)

The key things that I'd like to resolve here are:

• Restore the content around covering the Environment Webhooks
• Remove the 'Audit Log Webhooks' section from the Identity webhooks page
• Make the identity webhooks page more clear about the use case
• Add the payload to the Audit Log webhooks content and make it more clear which parts of he page refer to the webhook, and which refer to the audit log itself