ext-typo3: Remove/fix WordPress-specific content carried over from ext-wp

[toderash]

The ext-typo3.md spec was adapted from ext-wp.md and retains several
WordPress-specific items that are incorrect or irrelevant for TYPO3.

Terminology

• The "Plugin Package Type" section heading should be "Extension Package Type"
  to match TYPO3 terminology (the type itself is already correctly named
  typo3-extension).
• Line in the package artifact description reads "installable PHP plugin" —
  should be "installable PHP extension".
• Banner artifact description says "plugin listing pages" — should be
  "extension listing pages".

Banner dimensions

The sizes 772x250 and 1544x500 are WordPress.org-specific. TYPO3's
extension repository does not use these dimensions. This sentence should be
removed or replaced with TYPO3-appropriate guidance once a convention is
established.

Section aliases

The following aliases exist in the spec to support legacy WordPress readme.txt
field names:

• change_log → changelog
• frequently_asked_questions → faq
• screenshot → screenshots

These have no equivalent convention in TYPO3. They should be removed unless
there is a TYPO3 source that uses these names.

Copy-paste bug in banner and screenshot sections

Both sections contain: "Clients MAY conditionally show icons based on the
user's language." The word "icons" should be "banners" and "screenshots"
respectively. (The same bug exists in ext-wp.md and should be fixed there
too.)

Undocumented fields in Common section

The Common section contains an informal JSON snippet with three undocumented
fields (keywords, compatibility, typo3.extension-key) with no prose
explaining their meaning, location in the Metadata Document, or constraints.
This should be replaced with proper spec language.