docs: convert Component.add docstring to Google Style

[mvanhorn]

Summary

Contributes to #1072.

Converts the Component.add docstring in src/icalendar/cal/component.py from reStructuredText :param: / :returns: syntax to Google Style with Parameters:, Returns (omitted, returns nothing), and an Example: block. Follows the pattern set by #1316 (string_parameter) and #1309 (caselessdict).

The Example block adds a runnable doctest that verifies Component.add correctly stores the value as a vText (Team sync → vText(b'Team sync')).

Also adds a CHANGES.rst entry under the Documentation section.

Why these changes

The original docstring used :param name: ... :type name: string :returns: None. The project's style guide now expects Google Style headings and inline type information. This PR is one more step toward the consistent format described in #1072.

Verification

• python3 -m doctest src/icalendar/cal/component.py (run from src/): the new Component.add example block passes. (Three unrelated pre-existing failures in links, to_jcal, and uid are about hardcoded UIDs/timestamps and are not touched by this PR.)
• No code behavior change — docstring only.

Documentation preview

The Read the Docs preview will be linked by the bot after this PR is opened. The function shows up at icalendar.cal.component.Component.add.

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📚 Documentation preview 📚: https://icalendar--1390.org.readthedocs.build/

[read-the-docs-community]

Documentation build overview

📚 icalendar | 🛠️ Build #32715840 | 📁 Comparing 2ccd9bc against latest (c343662)

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4 files changed

± 404.html
± reference/changelog.html
± reference/api/icalendar.cal.component.html
± _modules/icalendar/cal/component.html