feat: add the tab_footnote() method

[rich-iannone]

This PR adds the tab_footnote() method which allows you to add footnotes for different locations in the table. The method integrates footnote mark placement into the rendering pipeline for table headings, column labels, body cells, etc., and extends the internal data structures and options to support footnote config.

Here's an example of how this works in practice:

import polars as pl
from great_tables import GT, loc, md, html
from great_tables.data import towny

tbl_data = (
    pl.from_pandas(towny)
    .filter(pl.col('csd_type') == 'city')
    .select(['name', 'density_2021', 'population_2021', 'density_2016', 'population_2016'])
    .top_k(8, by='population_2021')
    .sort('population_2021', descending=True)
    .with_columns([
        ((pl.col('population_2021') - pl.col('population_2016')) / pl.col('population_2016') * 100).round(1).alias('pop_change'),
        ((pl.col('density_2021') - pl.col('density_2016')) / pl.col('density_2016') * 100).round(1).alias('density_change')
    ])
)

(
    GT(tbl_data, rowname_col='name')
    .tab_header(
        title=md('Ontario Cities: **2016 vs 2021 Census Data**'),
        subtitle='Comparison of population and density changes over 5 years.'
    )
    .tab_stubhead(label='Municipality')
    .tab_spanner(label='2016 Census', columns=['population_2016', 'density_2016'])
    .tab_spanner(label='2021 Census', columns=['population_2021', 'density_2021'])
    .tab_spanner(label='5-Year Change', columns=['pop_change', 'density_change'])
    .fmt_integer(columns=['population_2016', 'population_2021'])
    .fmt_number(columns=['density_2016', 'density_2021'], decimals=1)
    .fmt_number(columns=['pop_change', 'density_change'], decimals=1)
    .cols_label(
        population_2016='Population',
        density_2016='Density',
        population_2021='Population',
        density_2021='Density',
        pop_change='Population (%)',
        density_change='Density (%)'
    )
    .tab_footnote(
        footnote='Data taken from the census.',
        locations=loc.title()
    )
    .tab_footnote(
        footnote='Municipality names as they appear in the census.',
        locations=loc.stubhead()  # Test stubhead footnote
    )
    .tab_footnote(
        footnote='Population and density figures from the 2016 Census.',
        locations=loc.spanner_labels(ids=['2016 Census'])
    )
    .tab_footnote(
        footnote='Population and density figures from the 2021 Census.',
        locations=loc.spanner_labels(ids=['2021 Census'])
    )
    .tab_footnote(
        footnote='Percentage change calculated as ((2021 - 2016) / 2016) × 100.',
        locations=loc.spanner_labels(ids=['5-Year Change'])
    )
    .tab_footnote(
        footnote='Density measured in persons per square kilometer.',
        locations=loc.column_labels(columns=['density_2016', 'density_2021'])
    )
    .tab_footnote(
        footnote='Part of the Greater Toronto Area.',
        locations=loc.stub(rows=['Toronto', 'Brampton', 'Mississauga'])
    )
    .tab_footnote(
        footnote='Highest population growth in this dataset.',
        locations=loc.body(columns='pop_change', rows=3)
    )
    .tab_source_note(
        source_note = md("Data taken from the `towny` dataset (originally from the **gt** package).")
    )
)

And here is how it looks:

Fixes: #164

[codecov]

Codecov Report

❌ Patch coverage is 92.15116% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.70%. Comparing base (5196b53) to head (6b58300).

Files with missing lines	Patch %	Lines
great_tables/_utils_render_html.py	92.59%	18 Missing ⚠️
great_tables/_locations.py	89.70%	7 Missing ⚠️
great_tables/_footnotes.py	92.30%	1 Missing ⚠️
great_tables/_text.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #763      +/-   ##
==========================================
+ Coverage   91.43%   91.70%   +0.26%     
==========================================
  Files          47       47              
  Lines        5547     5868     +321     
==========================================
+ Hits         5072     5381     +309     
- Misses        475      487      +12     

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