Support URLs to arbitrary Flagsmith instances

### Is your feature request related to a problem? Please describe.

Currently, there's no mechanism to reference a Flagsmith URL generically without referencing the specific instance as well. For example, https://app.flagsmith.com/account is the Account page on the public SaaS Flagsmith instance, but I have no way of pointing someone to that page without knowing their self-hosted instance URL.

This makes writing documentation or sharing links to customers more difficult, as it requires knowing the actual instance URL which is often not known or reachable from outside the customer's network. This is why we have instructions like "go to Project -> Settings" instead of having a clickable link that works for both SaaS and self-hosted.

Some URLs have instance-specific parameters such as organisation/project/environment IDs. To handle these cases, they could be provided as placeholders that default to the current/last-visited organisation/project/environment.

Example: https://my.flagsmith.com/project/_project_/environment/_environment_/features could end up redirecting to https://app.flagsmith.com/project/18444/environment/UgLNVEFpLooRAwiwLdY8bR/features

### Describe the solution you'd like.

I like Home Assistant's solution for this, which is to use a dedicated website that stores your instance URL locally: https://my.home-assistant.io

For example, when I visit https://my.home-assistant.io/redirect/helpers/, this is what I see:


I then get redirected to https://hass.my-domain/config/helpers

Home Assistant has a fixed list of available redirects with optional parameters, available here: https://github.com/home-assistant/my.home-assistant.io/blob/main/redirect.json

Source code available here https://github.com/home-assistant/my.home-assistant.io

### Describe alternatives you've considered

🤷

### Additional context

_No response_