docs: guard fetwfe()/betwfe() bacondecomp examples with requireNamespace (#260)

[gregfaletto]

Refs #260.

fetwfe() and betwfe()'s primary runnable @examples opened with a bare
library(bacondecomp) + data(castle). bacondecomp is a Suggests-only
dependency, so example(fetwfe) / example(betwfe) errored with
there is no package called 'bacondecomp' for any user without it installed.
(R CMD check --as-cran installs Suggests, so the check itself was always clean —
this only bit interactive example() use.)

Fix

Wrap each of the two affected example bodies in
if (requireNamespace("bacondecomp", quietly = TRUE)) { ... }. With bacondecomp
present (including under R CMD check) the example runs exactly as before; without
it, the example is silently skipped instead of erroring.

Scope — deliberately just two examples

Only fetwfe() and betwfe() have un-guarded, runnable bacondecomp examples.
The other estimator examples are intentionally left untouched:

• etwfe()'s bacondecomp example is already inside \dontrun{} (never runs → no
  problem).
• fetwfeWithSimulatedData() / etwfeWithSimulatedData() /
  betwfeWithSimulatedData() use genCoefs() / simulateData() and do not use
  bacondecomp at all, so they must NOT be gated on it.

(This is the narrowly-correct implementation of #260. The open drive-by PR #262
applied the bacondecomp guard to all six examples — including the three
simulateData-based ones that don't use bacondecomp — which is incorrect; this
PR supersedes it.)

Bookkeeping

• Version 1.26.0 → 1.26.1. The NEWS.md 1.26.1 section also documents the [docs] Clarify twfeCovs estimand wording (pooled, not separate) + genCoefs @param G "Optional" #257
  doc-clarity fix (twfeCovs estimand wording + genCoefs() @param G "Optional"),
  which merged via docs: clarify twfeCovs estimand wording and add 'Optional' to genCoefs @param G #259 without a NEWS entry.

Gate results

• air format .: clean.
• devtools::document(): regenerated only man/fetwfe.Rd + man/betwfe.Rd; no drift.
• devtools::test(): [ FAIL 0 | WARN 0 | SKIP 0 | PASS 3351 ].
• R CMD check --as-cran: 0 errors | 0 warnings | 0 notes (Status OK), fetwfe 1.26.1.

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