[DOCS] Reformat decimal digit token filter

[jrodewig]

Reformats the decimal digit token filter docs as part of #44726.

Changes

• Adds a title abbreviation
• Updates the description and adds a Lucene link
• Adds analyze and custom analyzer snippets

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-search (:Search/Analysis)

[romseygeek]

LGTM. It's odd that this is a tokenfilter rather than a charfilter, but that's a matter for lucene rather than a docs issue :)

It might be useful to add a token to the analysis example that mixes bengali numbers and text, to illustrate that the filter operates on characters and not just whole tokens.

[jrodewig]

Thanks @romseygeek. Good point re: mixing numbers and text. I'll update the analysis example accordingly.