feat: add plain text page support

[kof]

Description

Adds support for plain text pages. Related to #4257

This PR introduces text as a supported document type, stores plain text page content in page metadata, and generates text/plain route handlers for Remix and React Router templates.

It also limits HTML-specific Builder actions on text pages, excludes XML/text pages from the generated static sitemap, and adds SDK coverage for text document schema and sitemap behavior.

Steps for reproduction

1. Create or edit a page in Builder.
2. Set the document type to TEXT.
3. Enter plain text content in page settings.
4. Build with a Remix or React Router template.
5. Open the generated text page URL, for example /robots.txt.
6. Expect the response body to contain the configured text content with Content-Type: text/plain; charset=utf-8.
7. In Builder, expect HTML insert/wrap/convert/tag actions to be unavailable for text pages.

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 0000)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env file