fix outdated plugin doc filenames in docs/plugins/README

[ambikeesshh]

Problem

docs/plugins/README.md still describes the old layout (main.mdx, api-endpoints.mdx). On disk, plugins use overview.mdx and api.mdx (see docs/plugins/slack/).

Fix

Update the file tree, section titles, and Slack example links so they match the real filenames.

Touch

• docs/plugins/README.md

Done when

• no remaining main.mdx / api-endpoints.mdx references for the standard four pages
• examples point at overview.mdx / api.mdx
• optional pages section still makes sense

Verify

rg 'main\.mdx|api-endpoints\.mdx' docs/plugins/README.md
ls docs/plugins/slack/

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.