Start documenting diagnostics

[voltrevo]

Diagnostics are errors/lints/etc that the toolchain emits when processing input. Like this:

➜  ValueScript git:(main) vstc run inputs/passing/captureMutated.ts     
inputs/passing/captureMutated.ts:2:7: Lint: `x` should be declared using `const` because it is implicitly const due to capture
inputs/passing/captureMutated.ts:3:3: Error: Cannot mutate captured variable `x`

Failed with 1 error(s)

Every diagnostic should have an extended description. Start this process by creating docs/diagnostics/cap-const.md and docs/diagnostics/cap-mutate.md.

Include at least one small but complete program that generates the diagnostic on each diagnostic page.

Include something like this on each of these pages:

If you're having trouble resolving this error, you can search the issues for it or create a new one.