Document template variable replacements

[Copilot]

No documentation existed for PPTail's token substitution system, leaving template authors to reverse-engineer available variables from source code.

Adds a comprehensive variable replacements reference to README.md covering:

• Template types — maps each template file to its purpose
• Content item variables — all 18 substitution tokens available in item-level templates ({Title}, {Content}, {Author}, {ByLine}, {Description}, {Teaser}, {TeaserOrDescription}, dates, {Link}, {Permalink}, {PermalinkUrl}, {Tags}, {Hashtags}, {Categories}) with notes on XML encoding and format specifier behavior in syndication contexts
• Page-level variables — {Content}, {NavigationMenu}, {Sidebar}, {Title}
• Site settings variables — {SiteTitle}, {SiteDescription}, {ContactEmail}, {Copyright}, plus user-defined variables from SiteSettings.Variables
• Path variables — {PathToRoot} / {PathToSiteRoot} including URL-encoded variants (%7BPathToRoot%7D) for editor compatibility
• Page link variables — all link-generating tokens with their optional parameters:
  • Simple links ({HomePageLink}, {ArchivePageLink}, {ContactPageLink}, {FeedLink}) with custom link text via {HomePageLink:my text}
  • Content item links: {PageLink:slug|link text}, {PostLink:slug|link text}
  • Tag/search links: {SearchLink:tag|link text}
  • File/image embeds: {FileLink:filename|link text}, {ImageLink:path|alt text} with configurable folder paths via FilesFolder/ImagesFolder site variables
• Redirect variable — {Url} (Redirect template only)
• Full availability matrix — 35-variable × 12-template grid showing exactly where each token is substituted, derived directly from the processing paths in TemplateProcessor and StringExtensions