Consolidate etable vignettes

[grantmcdermott]

Hey @lrberge,

As discussed, here is my attempt to consolidate the two etable vignettes. I think it reads pretty nicely as a single document now, but feel free to push back.

Note that I also made a small number of mostly cosmetic changes to some files. E.g.,

• Fixed the accent on your name (Berge → Bergé) across all vignettes.
• Consolidated the pkgdown navbar so all vignettes appear under a single "Vignettes" dropdown.
• Various minor language/typo fixes (grammar, consistency, etc.)

For the consolidated vignette itself, the main structural changes are:

• Merged all content from etable_new_features.Rmd into exporting_tables.Rmd and deleted the former.
• Promoted "Highlighting coefficients" to its own top-level section (rather than being nested under "LaTeX extras").
• Reordered the LaTeX extras subsections so that adjustbox and page.width are adjacent (since the latter references the former).
• Used hidden setFixest_etable(markdown = TRUE) chunks to render the LaTeX tables as PNGs in the pkgdown site, with a guard to skip this during R CMD check (since the LaTeX toolchain isn't available there).
• Rewrote the intro, section transitions, and several subsection intros for better flow as a single document.

Passes R CMD check with 0 errors, 0 warnings (the 1 NOTE about src/Makevars is pre-existing).