Fix a format token returning literal instead of lowercase meridiem

[vedaant00]

What

DateTime.format("a") returns the literal string "a" instead of the lowercase am/pm marker:

import pendulum
dt = pendulum.datetime(2020, 1, 1, 14, 0, 0)
dt.format("A")  # "PM"  (correct)
dt.format("a")  # "a"   (bug — expected "pm")

Why

The a token is declared in the formatter regex and in _LOCALIZABLE_TOKENS, but _format_localizable_token has a branch for A and none for a, so a falls through to return token.

Fix

• Add the missing a branch, mirroring A and lowercasing the localized day period (matching the existing _LOCALIZABLE_TOKENS["a"] lambda). Non-Latin locales like ja are unaffected since .lower() is a no-op there.
• Added test_am_pm_lowercase.
• Added the a row to the format-token docs table.

Full suite passes (1838 passed).

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

[vedaant00]

Closing as a duplicate of #984, which predates this by a few weeks. Left a note there about also adding the a row to the format-token docs table (the one extra thing this PR covered).