error-index is slow to load and shows docs for wrong item while loading

[jsha]

Steps to reproduce:

1. In an IDE using rust-analyzer, write code with an error
2. In the resulting error message, click the link for more details of the error message.

Expected result:

Page shows the appropriate error message.

Actual result:

Page briefly shows E0001 while loading, then jumps to the correct error message:

This is because the error index has gotten quite large. The highest error code is E0788. The total page size is 1.5 MB. The number of DOM elements is 29,342 (browser engineers recommend < 1,500).

We should generate a separate page for each error message, and start linking to that, while still generating error-index.md so links from older tooling remain valid and up-to-date.

This may also help those individual pages rank more highly when someone searches for error text. Right now the page is so large that search engines don't consider it a good match even when someone searches for exact text of a specific error message.

[GuillaumeGomez]

Taking a look.