Add log2 and log10 functions to STDLIB documentation

[Shawon-Dey188]

What does this PR do?

Checklist

• make test passes locally
• New builtins have signature comments and docs in docs/BUILTINS.md
• New library functions follow conventions in docs/STDLIB.md
• New examples have a comment header explaining what they demonstrate
• CHANGELOG.md updated (if user-facing change)

[InauguralPhysicist]

Closing as a duplicate of your own #553, which already adds these two rows to docs/STDLIB.md — house convention here is that docs for new library functions ride the same PR as the code (one reviewable unit, one CI cycle). Two details from this version are worth carrying over into #553 though: the capitalized descriptions ("Base-2 logarithm") and keeping the blank line after the table — #553's current doc hunk loses that line. Fold those in there and it's one clean PR.