R1:2fa904921e1a

Render object monograms and embedding references more gracefully when building a Remarkup table of contents

Summary:
Fixes T8845. Ref T13102. See PHI467. Currently, object monograms like `L1` which appear in Remarkup headers render incorrectly (with an internal placeholder "x") in the table of contents:

{F5475505}

Instead, render them down to just, e.g., `L1` in plain text.

For `{P123}` I just rendered it to `{P123}` since it's not really clear to me what users intend. This could be adjusted if there's…