docs: add Replacements-stripping recipe + example for digit false positives

[jollycurv-e]

Hi, I'm jollycurv-e, I run ForestBot. I've been using rustrict as the censorship backend and ended up with this solution to a false positive on one of the commands. Figured it was worth sending upstream.

What

Adds a documented recipe (README) + runnable example (examples/leetspeak_replacements.rs) for removing
specific digit-to-letter mappings from the default Replacements table, for cases where digit-heavy text
(zip codes, numeric IDs, prices) is decoding into flagged words via leetspeak-style substitution.

Why

No code changes to the crate itself. This is purely additive documentation/example, using the existing
public Replacements::customize_default() API. Motivated by a real production false positive: a chat bot
feature let users look up gas prices by zip code, and the zip codes themselves were getting flagged via
digit-substitution decoding (e.g. 9 → g, 0 → o).

Testing

examples/leetspeak_replacements.rs builds and runs clean locally:

cargo run --example leetspeak_replacements --features customize