DOC: move all spreadsheet screenshots into a common folder

Author: afeld

doc/source/_static/excel_conditional.png renamed to doc/source/_static/spreadsheets/conditional.png

@@ -160,7 +160,7 @@ Filtering
 
 `In Excel, filtering is done through a graphical menu. <https://support.microsoft.com/en-us/office/filter-data-in-a-range-or-table-01832226-31b5-4568-8806-38c37dcc180e>`_
 
-.. image:: ../../_static/excel_filter.png
+.. image:: ../../_static/spreadsheets/filter.png
    :alt: Screenshot showing filtering of the total_bill column to values greater than 10
    :align: center
 
@@ -177,7 +177,7 @@ In spreadsheets, logical comparison can be done with `conditional formulas
 We'd use a formula of ``=IF(A2 < 10, "low", "high")``, dragged to all cells in a new ``bucket``
 column.
 
-.. image:: ../../_static/excel_conditional.png
+.. image:: ../../_static/spreadsheets/conditional.png
    :alt: Screenshot showing the formula from above in a bucket column of the tips spreadsheet
    :align: center
 
@@ -249,7 +249,7 @@ Sorting by values
 
 Sorting in spreadsheets is accomplished via `the sort dialog <https://support.microsoft.com/en-us/office/sort-data-in-a-range-or-table-62d0b95d-2a90-4610-a6ae-2e545c4a4654>`_.
 
-.. image:: ../../_static/excel_sort.png
+.. image:: ../../_static/spreadsheets/sort.png
    :alt: Screenshot of dialog from Excel showing sorting by the sex then total_bill columns
    :align: center
 
@@ -283,7 +283,7 @@ The `FIND
 <https://support.microsoft.com/en-us/office/find-findb-functions-c7912941-af2a-4bdf-a553-d0d89b0a0628>`_
 spreadsheet function returns the position of a substring, with the first character being ``1``.
 
-.. image:: ../../_static/excel_sort.png
+.. image:: ../../_static/spreadsheets/sort.png
    :alt: Screenshot of FIND formula being used in Excel
    :align: center
 
@@ -378,7 +378,7 @@ let's find the average gratuity by size of the party and sex of the server.
 
 In Excel, we use the following configuration for the PivotTable:
 
-.. image:: ../../_static/excel_pivot.png
+.. image:: ../../_static/spreadsheets/pivot.png
    :alt: Screenshot showing a PivotTable in Excel, using sex as the column, size as the rows, then average tip as the values
    :align: center
 

doc/source/_static/excel_filter.png renamed to doc/source/_static/spreadsheets/filter.png

@@ -626,7 +626,7 @@ the pandas-equivalent operations compared to software you already know:
                 </div>
                 <div class="col-lg-6 col-md-6 col-sm-6 col-xs-12 d-flex">
                     <div class="card text-center intro-card shadow">
-                    <img src="../_static/logo_excel.svg" class="card-img-top" alt="Excel logo" height="52">
+                    <img src="../_static/spreadsheets/logo_excel.svg" class="card-img-top" alt="Excel logo" height="52">
                     <div class="card-body flex-fill">
                         <p class="card-text">Users of <a href="https://en.wikipedia.org/wiki/Microsoft_Excel">Excel</a>
                             or other spreadsheet programs will find that many of the concepts are transferrable to pandas.</p>