add links to 2_21 release; updated script add_links.sh

Author: mitzimorris

docs/2_21/functions-reference/CSR.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/CSR.html">view current version</a>.
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 <div id="CSR" class="section level2">
 <h2 number="6.1"><span class="header-section-number">6.1</span> Compressed Row Storage</h2>
 <p>Sparse matrices are represented in Stan using compressed row storage

docs/2_21/functions-reference/Phi-function.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/Phi-function.html">view current version</a>.
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 <div id="Phi-function" class="section level2">
 <h2 number="3.12"><span class="header-section-number">3.12</span> Probability-Related Functions</h2>
 <div id="normal-cumulative-distribution-functions" class="section level3">

docs/2_21/functions-reference/absolute-functions.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/absolute-functions.html">view current version</a>.
+</div>
 <div id="absolute-functions" class="section level2">
 <h2 number="2.2"><span class="header-section-number">2.2</span> Absolute Functions</h2>
 <!-- R; abs; (T x); -->

docs/2_21/functions-reference/appendix.html

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 <div id="appendix" class="section level1">
 <h1 class="unnumbered" number=""><i style="font-size: 110%; color:#990017;">Appendix</i></h1>
 

docs/2_21/functions-reference/argument-order-and-the-vertical-bar.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/argument-order-and-the-vertical-bar.html">view current version</a>.
+</div>
 <div id="argument-order-and-the-vertical-bar" class="section level2">
 <h2 number="10.2"><span class="header-section-number">10.2</span> Argument Order and the Vertical Bar</h2>
 <p>Each probability function has a specific outcome value and a number of

docs/2_21/functions-reference/array-broadcasting.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/array-broadcasting.html">view current version</a>.
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 <div id="array-broadcasting" class="section level2">
 <h2 number="4.3"><span class="header-section-number">4.3</span> Array Broadcasting</h2>
 <p>The following operations create arrays by repeating elements to fill

docs/2_21/functions-reference/array-concatenation.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/array-concatenation.html">view current version</a>.
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 <div id="array-concatenation" class="section level2">
 <h2 number="4.4"><span class="header-section-number">4.4</span> Array Concatenation</h2>
 <!-- T; append_array; (T x, T y); -->

docs/2_21/functions-reference/array-operations.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/array-operations.html">view current version</a>.
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 <div id="array-operations" class="section level1">
 <h1 number="4"><span class="header-section-number">4</span> Array Operations</h1>
 <ul>

docs/2_21/functions-reference/array-reductions.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/array-reductions.html">view current version</a>.
+</div>
 <div id="array-reductions" class="section level2">
 <h2 number="4.1"><span class="header-section-number">4.1</span> Reductions</h2>
 <p>The following operations take arrays as input and produce single

docs/2_21/functions-reference/array-size-and-dimension-function.html

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+This is an old version, <a href="https://mc-stan.org/docs/functions-reference/array-size-and-dimension-function.html">view current version</a>.
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 <div id="array-size-and-dimension-function" class="section level2">
 <h2 number="4.2"><span class="header-section-number">4.2</span> Array Size and Dimension Function</h2>
 <p>The size of an array or matrix can be obtained using the <code>dims()</code>