docs: add endpoints for story creation and editing

[ZhiFenBL]

Document Wattpad write & edit endpoints

Summary

This PR expands the documentation beyond read-only endpoints to cover Wattpad's write and edit surface — story creation, part editing, tagging, media uploads, and the modern token-based authentication flow used by the Android app. It adds 24 new endpoint pages, 11 new data types, and updates several existing docs with newly discovered fields and parameters.

Because Wattpad's write operations largely run on the legacy apiv2 architecture (rather than the v3/v4/v5 REST APIs used for reads), this PR also documents the conventions that come with it — application/x-www-form-urlencoded bodies, and unnumbered error/error_count failure responses instead of the standard numbered error codes.

What's included

Write / Edit endpoints (legacy apiv2)

• Create Story (POST /apiv2/newstory) — create a new story group + first part, or append a part to an existing group
• Edit Part Content (POST /apiv2/editstory) — update a part's text, title, and inline media
• Edit Story Details (POST /apiv2/editstorygroup) — update story metadata and part ordering
• Edit Story Tags (POST /apiv2/editstorytags) — add/replace a story's tags
• Delete Story Group (POST /apiv2/deletestorygroup) — delete a story and all its parts
• Update Story Cover (POST /apiv2/updatestorycover) — Base64/form-encoded cover upload

Write endpoints (REST + writing service)

• Update Part Visibility (PUT /api/v3/story_parts/{id}) — publish/unpublish a part
• Upload Inline Image & Upload Part Image (POST /v5/media/...) — multipart/form-data image uploads
• Image Moderation (/v5/image_moderation/result) — confirm an uploaded image cleared moderation
• Story Part Polls (/writing/v1/.../polls) — read and create part polls

Authoring & discovery reads

• Sessions — modern token-based auth (now the preferred flow in Authentication)
• Writer Stories, Published Stories — creator-facing story lists
• Suggested Story Tags, Tag Autocomplete — tag suggestions for authors
• Languages, Taxonomies — system reference data for categorizing stories
• Part Text HTML — raw part HTML with paragraph IDs and inline-media attributes
• Home Feed, Premium Picks, User Subscriptions — additional discovery/account reads

Data types

Added: Session, GA, Media_Upload, Poll, Poll_Option, Taxonomy, Genre, Sub_Genre, Fandom.
Updated: Story (added story_text_url, paywall/rating/badge fields), Part (added hash, wordCount, dedication, scheduledPublishDatetime), Language (added enName, locale, code, categories).

Updates to existing docs

• Authentication — restructured into preferred (session token) vs. legacy (cookie) flows
• Parts — documented text-hash caching and conditional (If-None-Match / ETag) requests
• Story Info — documented deep field nesting, the drafts param, and bulk text download
• Content Settings — added api.wattpad.com host and new fields; Blocks — noted auth requirement
• README — added all new endpoints and data types in alphabetical order

Notes for reviewers

• All new pages follow the existing endpoint/data-type templates (URL / Fields / Special Syntax / Error Codes / Example Usage).
• Write endpoints are marked Requires authentication and cross-link to the relevant auth and data-type pages.
• No application code changes — documentation only.