✨feat: support privacy-respecting analytics

[welpo]

This PR adds support for three privacy-preserving analytics services:

• Plausible
• Umami
• Goatcounter

These are set up in the new [extra.analytics], like so:

[extra.analytics]
# Specify which analytics service you want to use.
# Supported options: ["goatcounter", "umami", "plausible"]
service = "umami"

# Unique identifier for tracking.
# For GoatCounter, this is the code you choose during signup.
# For Umami, this is the website ID.
# For Plausible, this is the domain name (e.g. "example.com").
# Note: Leave this field empty if you're self-hosting.
id = "yourID"

# Optional: Specify the URL for self-hosted analytics instances.
# For GoatCounter: Base URL like "https://stats.example.com"
# For Umami: Base URL like "https://umami.example.com"
# For Plausible: Base URL like "https://plausible.example.com"
# Leave this field empty if you're using the service's default hosting.
self_hosted_url = ""

Docs (Mastering tabi post + config and theme.toml) have been updated to reflect the new feature.

Testing

Only non-self-hosted setups have been tested.

Additional Context

This feature was inspired and discussed in Issue #172.