refactor: Strip site-specific framing from docgen templates

[khvn26]

Contributes to #201

Strip both docgen-events.md and docgen-metrics.md down to pure catalogue data. Consumers of flagsmith docgen {events,metrics} previously got a full page with hard-coded framing — frontmatter, intro prose, and a section heading. The OpenTelemetry link in the events header turned out to be a relative URL that only resolved correctly when the events page happened to live where the library author had put it; any move in the consumer's docs tree silently broke it.

The templates now emit only the per-event / per-metric ### ... entries. Consumers supply their own frontmatter and intro — either by writing an MDX page that imports the generated fragment as a partial, or by concatenating a hand-maintained header before the docgen output in their build.

BREAKING CHANGE

Consumers of docgen events / docgen metrics must now supply their own page header. The generated file alone is no longer a publishable docs page.

For the MDX partial pattern, rename the output to an underscore-prefixed file (Docusaurus treats _foo.md as a non-routed partial) and import it from a hand-authored .mdx:

---
title: Events
sidebar_label: Events
sidebar_position: 30
---

import EventCatalogue from './_events-catalogue.md';

Your intro prose here, linking to your OTel docs at the right path for your site.

## Event catalogue

<EventCatalogue />

Then in your Makefile:

poetry run flagsmith docgen events > path/to/_events-catalogue.md

How did you test this code?

Regenerated the two snapshot fixtures (test_docgen__events__runs_expected.txt, test_docgen__metrics__runs_expected.txt) via pytest --snapshot-update, then ran the tests normally to confirm they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.26%. Comparing base (ae79cc7) to head (4cee526).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #210   +/-   ##
=======================================
  Coverage   97.26%   97.26%           
=======================================
  Files         104      104           
  Lines        4459     4459           
=======================================
  Hits         4337     4337           
  Misses        122      122           

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