refactor(block-kit): remove nested rich_text block example in favor of standalone elements

The rich_text block example (blocks/rich_text) duplicated the standalone
rich-text element examples (elements/rich_text_*), which the docs reference
pages now pin to directly. Every nested exampleNN is reproduced by a
standalone element example, so this is pure deduplication with no loss of
demonstrated coverage:

- example01 -> rich_text_section
- example02/03 -> rich_text_list
- example04 -> rich_text_preformatted
- example05 -> rich_text_quote
- example06-13 (broadcast/color/channel/date/emoji/link/user/usergroup)
  -> the respective standalone element examples

The rich-text-block reference page is prose-only and references no example
file, so removing this changes no docs reference.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

Author: zimeg

block-kit/README.md

@@ -20,7 +20,6 @@ Read the [docs](https://docs.slack.dev/block-kit/) to learn concepts behind thes
 - **[Input](https://docs.slack.dev/reference/block-kit/blocks/input-block)**: Collects information from users via elements. [Implementation](./src/blocks/input.py).
 - **[Markdown](https://docs.slack.dev/reference/block-kit/blocks/markdown-block)**: Displays formatted markdown. [Implementation](./src/blocks/markdown.py).
 - **[Plan](https://docs.slack.dev/reference/block-kit/blocks/plan-block)**: Displays a collection of related tasks. [Implementation](./src/blocks/plan.py).
-- **[Rich text](https://docs.slack.dev/reference/block-kit/blocks/rich-text-block)**: Displays formatted, structured representation of text. [Implementation](./src/blocks/rich_text.py).
 - **[Section](https://docs.slack.dev/reference/block-kit/blocks/section-block)**: Displays text, possibly alongside elements. [Implementation](./src/blocks/section.py).
 - **[Table](https://docs.slack.dev/reference/block-kit/blocks/table-block)**: Displays structured information in a table. [Implementation](./src/blocks/table.py).
 - **[Task card](https://docs.slack.dev/reference/block-kit/blocks/task-card-block)**: Displays a single task, representing a single action. [Implementation](./src/blocks/task_card.py).