Docs: Add sitemap.xml and robots.txt

[sequba]

Context

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.

[sequba]

Created by mistake

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.17%. Comparing base (e8f338c) to head (301db4c).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1600   +/-   ##
========================================
  Coverage    97.17%   97.17%           
========================================
  Files          170      170           
  Lines        14785    14785           
  Branches      3245     3245           
========================================
  Hits         14368    14368           
  Misses         409      409           
  Partials         8        8           

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[github-actions]

Performance comparison of head (301db4c) vs base (e8f338c)

                                     testName |   base |   head | change
------------------------------------------------------------------------
                                      Sheet A | 515.45 | 509.03 | -1.25%
                                      Sheet B | 170.38 | 162.55 | -4.60%
                                      Sheet T | 147.43 | 143.85 | -2.43%
                                Column ranges | 505.07 | 506.67 | +0.32%
Sheet A:  change value, add/remove row/column |   14.4 |  14.75 | +2.43%
 Sheet B: change value, add/remove row/column |  135.3 | 137.61 | +1.71%
                   Column ranges - add column | 147.53 |  146.8 | -0.49%
                Column ranges - without batch | 432.54 | 443.52 | +2.54%
                        Column ranges - batch | 116.25 | 121.88 | +4.84%