Docs: Add sitemap and robots.txt

[sequba]

Context

This is a hotfix branch. Should be merged to both master and develop

robots:

User-agent: *
Allow: /

Sitemap: https://hyperformula.handsontable.com/sitemap.xml

sitemap conf:

hostname: 'https://hyperformula.handsontable.com',
exclude: ['/404.html'],
changefreq: 'weekly'

How did you test your changes?

local docs build, sitemap verified by Claude Code

Types of changes

• Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Related issues:

1. Fixes Docs: Add sitemap and robots.txt #1599

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement.
• My change is compliant with the OpenDocument standard.
• My change is compatible with Microsoft Excel.
• My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• My changes require a documentation update.
• My changes require a migration guide.

[github-actions]

Performance comparison of head (d5214f4) vs base (42ae583)

                                     testName |   base |   head | change
------------------------------------------------------------------------
                                      Sheet A | 507.13 | 504.72 | -0.48%
                                      Sheet B | 162.42 |  161.9 | -0.32%
                                      Sheet T | 141.83 | 143.76 | +1.36%
                                Column ranges | 501.68 |  501.3 | -0.08%
Sheet A:  change value, add/remove row/column |  14.28 |  13.78 | -3.50%
 Sheet B: change value, add/remove row/column | 129.43 | 131.35 | +1.48%
                   Column ranges - add column | 141.32 | 139.79 | -1.08%
                Column ranges - without batch | 425.96 | 419.63 | -1.49%
                        Column ranges - batch | 118.17 | 116.95 | -1.03%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.21%. Comparing base (42ae583) to head (d5214f4).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1601      +/-   ##
==========================================
+ Coverage   97.17%   97.21%   +0.04%     
==========================================
  Files         170      170              
  Lines       14747    14928     +181     
  Branches     3236     3300      +64     
==========================================
+ Hits        14330    14512     +182     
+ Misses        409      408       -1     
  Partials        8        8              

see 17 files with indirect coverage changes

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