TINYDOC-3524 - Fix empty table rows and add ignored-words persistence and custom-dictionary guidance in Spell Checker docs

[kemister85]

Ticket: TINYDOC-3524

Site: Staging branch

Changes:

• Removed the empty rows from the Spell Checker plugin page tables. The APIs, Events, and Commands tables each contained an empty spanning-cell row (3+| / 2+|) that rendered as a blank row; all three have been removed.
• Added a Persisting ignored words section to the spellchecker_ignore_list option. It explains that words ignored at runtime (via the Ignore / Ignore All menu items or the addIgnoredWords API) apply only to the current session, and outlines how integrators can persist them across sessions by storing them (for example, when handling the SpellcheckerIgnore / SpellcheckerIgnoreAll events) and reapplying them through spellchecker_ignore_list on initialization. Behaviour validated against tinymce-premium source (plugins/tinymcespellchecker/src/main/ts/api/Api.ts), which confirms addIgnoredWords mutates only the in-memory ignore list and cache.
• Added a note to the Self-hosted Installation section stating that custom dictionaries are available only with a self-hosted spelling service and cannot be configured on Tiny Cloud, with links to the self-hosted deployment page and the custom dictionaries page.

Pre-checks:

• Branch is correctly prefixed: hotfix/8/TINYDOC-3524
• Files have been included where required (if applicable).
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed.