Support #custom-id in ToC

ReDoc can include headers (# Header, ## Sub Header or similar) in markdown into table of contents.
Link ids (#id-here) are automatically generated from text in header.

I think it would be benefical if user can customize link ids using custom id extension, so

• it is table when text are changed (e.g. fixed typo).
• it can be consistent in translated API Docs (see below):

// In English page
# API Convention {#api-convention}

// In Japanese page
# API の慣習 {#api-convention}

// In Russian page
# API условность {#api-convention}

Start by locating the markdown heading and table-of-contents generation path in the TypeScript codebase. Check how headings currently receive generated link ids, then verify that custom ids such as {#api-convention} are preserved in the table of contents across translated headings.